Media playback is unsupported on your device Media caption The crowds are shouting "Russia without Putin", reports the BBC's Sarah Rainsford from the Moscow march 
 
 Tens of thousands of people have marched through central Moscow to honour opposition politician Boris Nemtsov, who was shot dead on Friday. 
 
 They carried portraits of Mr Nemtsov and banners saying "I am not afraid". 
 
 He had been due to lead an opposition march on Sunday but his killing turned the event into a mourning rally. 
 
 Mr Nemtsov's allies have accused the Kremlin of involvement, but President Vladimir Putin condemned the murder as "vile" and vowed to find the killers. 
 
 Opposition supporters gathered at a point not far from the Kremlin before marching past the spot on Great Moskvoretsky Bridge where Mr Nemtsov was killed. Some chanted "Russia without Putin!" 
 
 Several thousand people also marched in St Petersburg. 
 
 Flags and flowers 
 
 Moscow city authorities had approved a march of up to 50,000 people there. 
 
 Image copyright AFP Image caption The attendance has yet to be confirmed but runs into the tens of thousands 
 
 Image copyright EPA Image caption There is a heavy police presence, including at the scene of the killing 
 
 There has been no official figure on turnout yet, with estimates ranging from 16,000 to 70,000. 
 
 Many people carried the national flag and flowers to lay at the scene of the killing, which was already piled with tributes. A few were holding Ukrainian flags. 
 
 Some of the placards read: "He died for the future of Russia" and "They were afraid of you, Boris". 
 
 Maria Glazacheva, a 22-year-old student in Moscow who joined the march, told the BBC: "Boris was a good and honest man. Moscow is a sad place today." 
 
 At the scene: Sarah Rainsford, BBC News, Moscow 
 
 The mood was sombre as the crowd gathered. Many were carrying flowers to place at the spot where Boris Nemtsov was killed. They had placards displaying his photograph, and slogans that included a play on Mr Nemtsov's name so that it read: "Fight on!" 
 
 As the crowd turned the corner and flooded on to the embankment, they began to chant. Quietly at first, but more loudly as they approached the Kremlin: "Russia without Putin" and "Putin, leave!" 
 
 These were extraordinary scenes right at the heart of Russia's capital. The police were out in force, but the march was peaceful. Many in the crowd were carrying their national flag, with a black ribbon tied to it. 
 
 They said they wanted to show that they are patriots too. That criticising those in power does not make you an enemy of the state. People told me that - like Boris Nemtsov - they just want to see a different Russia. 
 
 Who was Boris Nemtsov? 
 
 Nemtsov killing polarises society 
 
 Russian and world reaction 
 
 Opposition politician Ilya Yashin told Associated Press the killing was "a political murder aimed at frightening the... part of the population that supported Nemtsov and did not agree with the government. I hope we won't get scared". 
 
 Another opposition figure, Gennady Gudkov, told Reuters: "If we can stop the campaign of hate that's being directed at the opposition, then we have a chance to change Russia. If not, then we face the prospect of mass civil conflict." 
 
 Pro-Kremlin activists from the group Anti-Maidan had said they would not disrupt the march. 
 
 'Contract hit' 
 
 Media playback is unsupported on your device Media caption Mr Nemtsov called for "honest elections" hours before he was killed 
 
 Russia's Investigative Committee said it was looking into a number of possible motives for Mr Nemtsov's murder. 
 
 They include his opposition to the Ukraine conflict, Islamic extremism - Mr Nemtsov had Jewish ancestry although he had become Orthodox Christian - and an opposition "sacrifice" of its leader to destabilise the state and undermine the president. 
 
 Presidential spokesman Dmitry Peskov said Mr Putin had noted "that this cruel murder has all the makings of a contract hit and is extremely provocative". 
 
 The investigators offered a reward of three million roubles ($48,000) for information leading to the killers. 
 
 Image copyright EPA Image caption A rally was also held in St Petersburg. The banner says: "Boris Nemtsov - we won't forget, we won't forgive". 
 
 Mr Yashin confirmed that Mr Nemtsov was preparing documents on Russian military involvement in Ukraine before his death. 
 
 Mr Yashin said: "For him the conflict between Ukraine and Russia was a real personal tragedy and he was trying to find any way to stop it. And he was trying to find proof that Mr Putin is sending our troops to Ukraine." 
 
 Restricted movement 
 
 Mr Nemtsov, 55, had been dining at a restaurant with his girlfriend Anna Duritskaya on Friday night. 
 
 They left together to walk to his flat, crossing the bridge, where a white car drew up and Mr Nemtsov was shot four times with a pistol at around 23:40 (20:40 GMT). 
 
 Ms Duritskaya has expressed a desire to return to her home country, Ukraine, but her lawyer said her movement had been restricted. Officials said she could be subject to "additional investigative action". 
 
 In a telegram to Mr Nemtsov's mother, published on the Kremlin website, Mr Putin vowed to bring the killers to justice. He praised Mr Nemtsov's openness and honesty. 
 
 Mr Nemtsov served as first deputy prime minister under President Boris Yeltsin in the 1990s but fell out of favour with Mr Putin and became an outspoken opponent. 
 
 Violent deaths of Putin opponents 
 
 April 2003 - Liberal politician Sergey Yushenkov assassinated near his Moscow home 
 
 July 2003 - Investigative journalist Yuri Shchekochikhin died after 16-day mysterious illness 
 
 July 2004 - Forbes magazine Russian editor Paul Klebnikov shot from moving car on Moscow street, died later in hospital 
 
 October 2006 - Investigative journalist Anna Politkovskaya shot dead outside her Moscow apartment 
 
 November 2006 - Former Russian spy Alexander Litvinenko died nearly three weeks after drinking tea laced with polonium in London hotel 
 
 March 2013 -Boris Berezovsky, former Kremlin power broker turned Putin critic, found dead in his UK home 

 LONDON (AP) — The widow of murdered Russian spy Alexander Litvinenko thinks the Russian government may have been involved in the murder of leading opposition figure Boris Nemtsov. 
 
 Marina Litvinenko told BBC Radio on Sunday she thinks Nemtsov's slaying Friday night in Moscow seems to have been the Russian government's way of silencing critics of President Vladimir Putin, but offered no proof. She said the Russian government has become particularly aggressive since the Ukraine crisis began. She called Nemtsov's death "absolutely devastating." 
 
 Putin has condemned the killing of Nemtsov and said he will lead the official inquiry into it. 
 
 Marina Litvinenko has blamed the Russian government for the poisoning of her husband, who died in London in 2006 after ingesting radioactive tea. An inquiry into her husband's death is ongoing. 

 MOSCOW (AP) — Carrying flowers, portraits and signs that said "I am not afraid," more than 20,000 people somberly marched Sunday in Moscow to mourn opposition politician Boris Nemtsov, whose slaying on the streets of the capital has shaken Russia's beleaguered opposition. 
 
 People lay flowers during a march next to the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin, was gunned down on Friday, Feb. 27, 2015... (Associated Press) 
 
 People march with posters reading ' fight!' near the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin, was gunned down on Friday, Feb. 27,... (Associated Press) 
 
 People with Russian national flags march in memory of opposition leader Boris Nemtsov, who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, in Moscow, Russia, Sunday, March 1, 2015. Thousands... (Associated Press) 
 
 People march next to flowers and posters reading ' Propaganda kills!, Fight! ' near the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin,... (Associated Press) 
 
 A man ties a bunch of flowers with a ribbon in the colours of the Russian national fag at the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir... (Associated Press) 
 
 People carry a huge banner reading 'those bullets for everyone of us, heroes never die!' as they march in memory of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the... (Associated Press) 
 
 People march in memory of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, with portraits of him and words reading ' those bullets for everyone of us, heroes... (Associated Press) 
 
 Portraits and flowers are seen at the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin, was gunned down on Friday, Feb. 27, 2015 near the... (Associated Press) 
 
 People march in memory of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, with portraits of him and words reading 'he died for the freedom to Russia, heroes... (Associated Press) 
 
 A man prepares portraits of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, to carry on the march in Moscow, Russia, Sunday, March 1, 2015. Russian investigators,... (Associated Press) 
 
 People carry portraits of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, with words reading ' He fought for a free of Russia, He fought for our future !'... (Associated Press) 
 
 People carry portraits of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, with words reading ' those bullets for everyone of us, heros never die!' in Moscow,... (Associated Press) 
 
 Police officers walk with their sniffing dog checking the area prior a march in memory of opposition leader Boris Nemtsov, who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, in Moscow, Russia,... (Associated Press) 
 
 A woman lays flowers at the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin, was gunned down on Friday, Feb. 27, 2015 near the Kremlin,... (Associated Press) 
 
 People carry a huge banner reading 'those bullets for everyone of us, heroes never die!' as they march in memory of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the... (Associated Press) 
 
 People with Russian national flags march in memory of opposition leader Boris Nemtsov who was gunned down on Friday, near the Kremlin, with the Kremlin in the background in Moscow, Russia, Sunday, March... (Associated Press) 
 
 Flowers and pesters reading 'propaganda kills! Fight!' are seen at the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin, was gunned down... (Associated Press) 
 
 A woman lays flowers at the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin, was gunned down on Friday, Feb. 27, 2015 near the Kremlin,... (Associated Press) 
 
 People carry a huge banner reading 'those bullets for everyone of us, heroes never die!' as they march in memory of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the... (Associated Press) 
 
 Flowers and pesters reading ' propaganda kills!' are seen at the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin, was gunned down on Friday,... (Associated Press) 
 
 People carry portraits of opposition leader Boris Nemtsov, who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, with words reading ' he died for the future of Russia!' in Moscow, Russia, Sunday,... (Associated Press) 
 
 Russian President Vladimir Putin has marginalized and intimidated his political opponents, jailing some and driving others into exile, since mass anti-Putin protests swept Moscow in 2011 and 2012. Nemtsov, 55, was among the few prominent opposition figures who had refused to be cowed. 
 
 The mourners on Sunday marched to the bridge near the Kremlin where Nemtsov was gunned down shortly before midnight Friday. Chanting "we'll not forget, we'll not forgive," the crowd filled the road along the Moscow River embankment. Many waved Russian flags. 
 
 The mood was somber, with heavy security. Police helicopters flew overhead and police boats patrolled the river. 
 
 Ilya Yashin, a friend and fellow opposition leader, said he hoped the killing would not frighten people. 
 
 "Essentially it is an act of terror. It is a political murder aimed at frightening the population, or the part of the population that supported Nemtsov and did not agree with the government," Yashin told The Associated Press. "I hope we won't get scared, that we will continue what Boris was doing." 
 
 The march could energize the opposition, but it could also prove to be a brief expression of emotions that dissipates in a climate of fear. 
 
 Mikhail Kasyanov, a former prime minister who joined the opposition, told the crowd the killing should be a turning point for Russia "for the simple reason that people who before thought that they could quietly sit in their kitchens and simply discuss problems within the family, now will start reconsidering everything that's going on in our country." 
 
 Russia's federal investigative agency said it was looking into several possible motives for Nemtsov's killing. 
 
 The first possibility, the Investigative Committee said, was that the killing was aimed at destabilizing the political situation in Russia and Nemtsov was a "sacrificial victim for those who do not shun any method for achieving their political goals." 
 
 This suggestion echoed comments by Putin's spokesman and other Russian politicians that the attack was a "provocation" against the state. The consensus of political commentators on state television was that the killing served the interests of Russia's enemies. 
 
 Yashin, however, said Russia's leadership and specifically Putin bore full political responsibility for Nemtsov's death. 
 
 "It was President Putin who created this atmosphere of hate in our country, the atmosphere of intolerance, which one way or another materialized in the bullet that killed my friend Boris Nemtsov," Yashin said. 
 
 Kremlin propaganda had identified Nemtsov as among the leaders of a "fifth column," painting him and other opposition figures as traitors in the service of a hostile West. 
 
 As a former deputy prime minister and longtime politician, Nemtsov retained strong ties among Russia's political and business elite, making his killing additionally shocking. 
 
 He was killed just hours after a radio interview in which he denounced Putin's "mad, aggressive and deadly policy of war against Ukraine." Nemtsov was working on a report presenting evidence that he believed proved that Russian servicemen were fighting with the separatists in Ukraine, despite official denials. 
 
 Yashin said it was unlikely the report would ever be published because investigators who searched Nemtsov's apartment after his death took away his laptop. 
 
 TV Center, a station controlled by the Moscow city government, broadcast a poor-resolution video from one of its web cameras that it said shows Nemtsov and his female companion shortly before the killing. The station, which superimposed its own time code on the footage, circled figures that it said were Nemtsov and the woman walking across the bridge on a rainy night. A snowplow that moved slowly behind the couple obscured the view of the shooting. TV Center then circled what it said was the suspected killer jumping into a passing car. The authenticity of the video could not be independently confirmed. 
 
 Investigators said Sunday that they were again questioning the woman, Ukrainian citizen Anna Duritskaya, who was with Nemtsov when he was killed. LifeNews, a television station with ties to the security services, said Duritskaya told investigators that she was in shock and could not remember what the killer looked like or the car he was in. 
 
 The Interior Ministry said it was offering 3 million rubles (nearly $50,000) for information about the killing. 
 
 U.S. Secretary of State John Kerry said the United States had no intelligence on who was being the shooting. "The bottom line is we hope there will be a thorough, transparent, real investigation, not just of who actually fired the shots, but who, if anyone, may have ordered or instructed this or been behind this," Kerry said Sunday on ABC's "This Week." 
 
 Another mourning march for Nemtsov was held earlier Sunday in St. Petersburg, drawing what police estimated was 6,000 people. 
 
 Nelly Prusskaya, a 66-year-old doctor, said she came to pay her respects to Nemtsov. 
 
 "I also came to say that I'm against the war in Ukraine," she said. "I'm against political murders." 
 
 Most Russians support Putin's policy in Ukraine. Since the annexation of Crimea a year ago, support for Putin has been more than 80 percent. 
 
 ___ 
 
 Irina Titova in St. Petersburg, Russia, contributed to this report.
– Boris Nemtsov was due to lead an opposition protest in Moscow today, and thousands of people are instead holding one in memory of the critic of Vladimir Putin who was gunned down on Friday, reports the BBC. Marchers carried signs declaring "He died for the future of Russia," "I am not afraid," and "They were afraid of you, Boris," reports the AP. "He was afraid of being killed," a fellow journalist who had recently met with Nemtsov tells the New York Times. "And he was trying to convince himself, and me, they wouldn’t touch him because he was a member of the Russian government, and they wouldn’t want to create a precedent." Many aren't hesitating to point the blame for Nemtsov's murder squarely at Putin, including Marina Litvinenko, widow of slain Russian spy Alexander Litvinenko. Per the AP, she thinks the Kremlin was probably also involved in Nemtsov's "absolutely devastating" death. "Boris had declared he would provide clear evidence of Russian armed forces' participation in [the war] in Ukraine. Somebody was afraid of this... They killed him," says Ukrainian President Petro Poroshenko, per the BBC. Putin himself has condemned the "cruel murder" that "has all the signs of a contract killing" and vowed "everything will be done so the perpetrators of a vile and cynical murder get the punishment they deserve."