Russian ex-spy Alexander Litvinenko has accused Russian President Vladimir Putin of involvement in his death, in a statement dictated before he died.
Mr Litvinenko, 43, who died in a London hospital on Thursday evening and is thought to have been poisoned, said his killer was "barbaric and ruthless".
Protest from around the world "will reverberate, Mr Putin, in your ears for the rest of your life," he said.
The Kremlin has dismissed allegations it was involved as "sheer nonsense".
Scotland Yard said officers were now investigating "an unexplained death".
Anti-terror police are leading the investigation, and it is still unclear what killed the former KGB agent.
Friends have said he was poisoned because of his criticism of Russia.
'Barbaric and ruthless'
In the statement, read out by his friend Alex Goldfarb outside University College Hospital, London, Mr Litvinenko said he had a "message to the person responsible for my present condition".
1 Nov - Alexander Litvinenko meets two Russian men at a London hotel and also meets Italian academic Mario Scaramella at a sushi bar in Piccadilly. Hours later he falls ill and is admitted to Barnet General Hospital
17 Nov - Mr Litvinenko is transferred to UCH
19 Nov - Reports say Mr Litvinenko is poisoned with thallium
21 Nov - A toxicologist says he may have been poisoned with "radioactive thallium"
22 Nov - Mr Litvinenko's condition deteriorates overnight. Thallium and radiation ruled out
23 Nov - The ex-spy dies in intensive care
"You may succeed in silencing me, but that silence comes at a price.
"You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed."
"The howl of protest from around the world will reverberate Mr Putin in your ears for the rest of your life," the statement added.
The statement was dictated on 21 November, when Mr Litvinenko realised he could die.
Mr Putin's spokesman Dmitry Peskov reiterated the Kremlin's earlier dismissal of allegations of involvement in the poisoning as sheer nonsense.
"Any death is always a tragedy," he said.
"Now it's up to the UK law enforcement agencies to investigate what happened."
After Mr Goldfarb had read out the statement, Mr Litvinenko's elderly father, Walter - who flew to the UK from Russia this week - said his son had been killed by a "tiny nuclear bomb".
"It was an excruciating death, he was taking it as a real man," he said.
"Even before his death, in such a state, he never lost his human dignity."
Mr Litvinenko had recently been investigating the murder of his friend, Russian journalist Anna Politkovskaya, another critic of the Putin government.
Russian dissident Oleg Gordievsky, a former KGB colonel and friend of Mr Litvinenko, maintained that the poisoning had been the work of the Russians.
The Russian security service had "sent a man with a poisonous pill to Britain", put a pill into Mr Litvinenko's tea and killed him, he told BBC News.
Intelligence analyst Glenmore Trenear Harvey said Mr Litvinenko had "made a lot of enemies" when he had been tasked with fighting corruption during his time with the Federal Security Service (FSB) - the KGB's successor.
Mr Harvey also said the poisoning could have been carried out by the "Russian mafia", made up of former-KGB men who had formed the group when the service broke up.
"So I think that while one could say they were trained by the KGB this is not in any way a Russian intelligence service hit," he told BBC News.
Before Mr Litvinenko's death, police said they suspected "deliberate poisoning" was behind his illness.
Investigators have been examining two meetings he had on 1 November - one at a London hotel with a former KGB agent and another man, and another rendezvous with Italian security consultant Mario Scaramella, at a sushi restaurant in London's West End.
Mr Litvinenko, who was granted asylum in the UK in 2000 after complaining of persecution in Russia, fell ill later that day.
In an interview with Friday's Telegraph newspaper, former KGB bodyguard Andrei Lugovoi said he had met Mr Litvinenko at the Millennium Hotel in Grosvenor Square but vigorously denied any involvement in the poisoning.
Mr Scaramella, who is involved in an Italian parliamentary inquiry into Russian secret service activity, said they met because he wanted to discuss an e-mail he had received.
Speaking in Friday's Times, film-maker Andrei Nekrasov said that, before he fell unconscious for the last time, his friend had told him: "I want to survive, just to show them. The bastards got me but they won't get everybody."