Bomb attacks on the British consulate and the HSBC bank headquarters in Istanbul have left at least 27 dead and up to 400 injured.
Hospitals have been treating hundreds of injured people
The top UK diplomat in the city, Consul-General Roger Short, was among at least 14 people killed in the attack on the consulate, its chaplain Ian Sherwood said.
The Turkish authorities say the attacks were carried out by suicide bombers, reportedly linked to al-Qaeda.
The explosions follow last Saturday's suicide bomb attacks against two synagogues in Istanbul that killed 25 people.
Responding to news of the blasts, UK Prime Minister Tony Blair said there could be "no holding back" in confronting the "menace" of global terrorism.
US President George Bush said the bombers had shown "utter contempt" for human life.
Istanbul was thrown into chaos by the twin blasts.
Traffic blocked ambulances as they tried to reach the wounded and much of the city's phone network was cut off.
The bomb at the consulate was so big, buildings hundreds of metres away had their windows blown out.
The BBC's Tim Franks, in Istanbul, says that hours after the event those shopkeepers who had not simply turned off the lights and gone home were trying to repair the damage.
A man who called the semi-official Anatolia news agency claimed that al-Qaeda and the Turkish Islamic militant group IBDA-C had jointly carried out the attacks.
Turkish Prime Minister Tayyip Erdogan vowed to track down those responsible for the bombings.
"Turkey will be like a fist [against the culprits]... The best response for us is to stay calm in the face of terrorism,"
he told a news conference.
The 15-storey headquarters of the London-based bank towers over a busy shopping area, which correspondents say would have been crowded when the explosion occurred at 1110 Turkish time (0910GMT).
Who are IBDA-C?
Islamic Great Eastern Raiders' Front
Armed, underground Islamist network
Claim latest bomb attacks
Linked to bomb attacks on two synagogues
Leader is in prison facing death penalty
The consulate bomb occurred two minutes later. The main building had been undergoing refurbishment and many staff were working in outbuildings in the complex.
The UK Foreign Office confirmed that British diplomatic staff member Lisa Hallworth had been killed in the blast as well as the Consul-General.
Turkish Foreign Minister Abdullah Gul said Turkey would not bow to terror.
"We are faced with organised terrorist attacks," he said. "This time it was British interests, last week it was two synagogues."
Murat Emre Duman was working in a building near the HSBC bank.
He said he saw "yellow smoke" coming out of the tower and "shattered glass from cars and windows everywhere".
The explosion killed the 28-year-old wife of a friend, he told BBC News Online.
"I saw her body. My friend doesn't even recognise me. He's in a state of shock."
Trading on the Istanbul stock exchange was suspended after the explosions and the Turkish lira dived on foreign exchange markets.
A group claiming to be linked to al-Qaeda also claimed responsibility for the synagogue attacks last Saturday.
It warned that new attacks against the US and its allies were being planned.
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Sixteen members of London's anti-terrorist police branch are to join the investigation into the latest attacks in Istanbul. UK Foreign Secretary Jack Straw is already heading to Istanbul.
The United States warned that more terrorist attacks were possible in Istanbul, closed its consulate and warned its citizens to stay away from the area where the bombings took place.
The Foreign Office in London issued the following number for people to call for information about those who may have been involved in the blasts: 020 7008 0000.