The synchronised blasts were over in 56 minutes. At 8:51 am a blast hits between Liverpool Street and Aldgate underground stations, killing seven people.
At 8:56 am an explosion detonates in the underground near the central King's Cross Station. A further 21 die.
At 9:17 am a device explodes at Edgware Road underground station, the western part of the centre of the city, slicing through a carriage, a wall and, police say, into two other trains, killing seven people.
At 9.47 am an explosion blows apart the number 30 double-decker bus from northern London's Hackney to central Marble Arch. Two die.
Police said the total came to 37 confirmed deaths and total casualties at 700. Of the injured, 300 were taken to the hospital by ambulance and the rest made their own way there. Witnesses related horrific scenes.
"There was a loud bang and the train ground to a halt. People started panicking, screaming and crying as smoke came into the carriage," said Arash Kazerouni, 22, who had been travelling from Liverpool Street, which serves the main financial district.
"It was terrible. The bus went to pieces. There were so many bodies on the floor," said Ayobami Bello, 46, a security guard at the nearby London School of Tropical Hygiene and Medicine.
"I can't believe it, I can't even believe I survived it," he said. "There was panic and everyone was running for their lives. I saw a lady coming towards me soaked in blood. Everyone was in confusion."
British Prime Minister Tony Blair interrupted a summit of the Group of Eight most powerful leaders in Gleneagles, Scotland, and flew to London to address the nation where he blamed terrorists.
"They should not and they must not succeed," the visibly shaken British leader said.
"When they try to intimidate us we will not be intimidated. When they seek to change our country or our way of life by these methods we will not be changed. When they try to divide our people or weaken our resolve, we will not be divided and our resolve will hold firm."
Blair promised the most intense police and security action "to make sure we bring those responsible to justice."
A previously unknown group, which calls itself the Organisation of al Qaeda Jihad in Europe, claimed it caused the blasts and threatened similar attacks against Italy, Denmark and other "Crusader" states with troops in Iraq and Afghanistan.
In a statement posted on the Internet, but which could not be authenticated, it said: "Heroic Mujahideen carried out a sacred attack in London, and here is Britain burning in fear, terror and fright in the north, south, east and west."
The group said the attacks were "in response to the massacres carried out by Britain in Iraq and Afghanistan."
"We have repeatedly warned the government and people of Britain, and we have now fulfilled our promise and have carried out a sacred military attack in Britain," it said. Blair indicated an Islamic group might be responsible.
"We know that these people act in the name of Islam, but we also know that the vast and overwhelming majority of Muslims here and abroad are decent and law abiding people who abhor terrorism every bit as much as we do," he said.
A stunned calm reigned in London, with hundreds of thousands of people forced to walk because of the underground shutdown and withdrawal of buses in the city centre.
At the G-8 Summit in Scotland, briefly abandoned by Blair, leaders vowed support for Britain. "We condemn utterly these barbaric attacks. We send our profound condolences to the victims and their families.
US President George W. Bush vowed to press his "war on terror".
"They have such evil in their heart that they will take the lives of innocent folks. The war on terrorism is on," Bush told reporters at the summit venue, the luxury golf resort of Gleneagles. Blair vowed to return and complete the G-8 Summit.
In the United States, Homeland Security Secretary Michael Chertoff said he had raised the threat level for mass transit and train systems to code orange or 'high'.
Security was stepped up in New York and Washington, still jittery after the September 2001 attacks on the United States by the al Qaeda terror network, while the nation-wide US rail system, Amtrak, also said it raised its security alert level.
Nato Chief Jaap de Hoop Scheffer called an extraordinary meeting of the body's policy-making North Atlantic Council for Friday to discuss possible 'measures', a spokesman said.
Following the September 11, 2001 attacks in the United States, Nato invoked Article Five of its charter, which says that an attack on one member country is deemed an attack against all members.
The London attacks triggered global security fears, and London and Paris stock markets began to tumble.
Governments across Europe beefed up security in airports, rail stations and public transport systems.
In Singapore, the chief executive of London's victorious 2012 Games bid, Keith Mills, said Olympic celebrations would be put on hold. "It's terribly unfortunate in terms of timing and clearly all our celebrations of the result yesterday will have to be put on hold," he said.
France, which lost out to Britain in the bid to host the 2012 Olympics, raised its anti-terror alert to red, the second highest rating, while Italy convened a meeting of top police and anti-terrorism chiefs in Rome.
In Spain, the attacks recalled that country's worst terrorist attack on March 11, 2004, in which 191 were killed and 1,900 injured in a series of train bombings in Madrid blamed on al Qaeda.