London ? The first bomb went off at 8:51 a.m., on a London Underground train just outside the financial district. Five minutes later, another train exploded, then another, and finally a crowded red double-decker bus. In 56 minutes, a city fresh from a night of Olympic celebrations was enveloped in eerie, blood-soaked quiet.

Terror had struck the British capital at the start of a busy work day, just as it had a year ago in Madrid and in 2001 in New York and Washington.

London police said they could confirm at least 37 people had been killed and 700 injured Thursday in the worst attack on London since the blitz in World War II.

The French Interior Minister Nicolas Sarkozy said the death toll was 50, citing a conversation with his British counterpart, and Australian Prime Minister John Howard on Friday said the toll was 52; he did not disclose the source of his information.

British Prime Minister Tony Blair blamed Islamic extremists and said the bombings were designed to coincide with the opening in Scotland of a G-8 summit of the world’s most powerful leaders. Foreign Secretary Jack Straw said the bombings – which came the day after London won the bid to host the 2012 Olympics – have the “hallmarks of an al-Qaida-related attack.”

Police said there had been no warning. Authorities initially blamed a power surge but realized it was a terror attack after the bus bombing near the British Museum at 9:47 a.m. – less than an hour after the first explosion.

Trapped passengers in the Underground railway threw themselves on the floor, some sobbing. As subway cars quickly filled with smoke, people used their umbrellas to try to break the windows so that they could get air. Passengers emerged from the Underground covered with blood and soot. On the street, in a light rain, buses ferried the wounded, and medics used a hotel as a hospital.

A forensic officer walks next to the wreckage of a double decker bus with its top blown off and damaged cars scattered on the road at Tavistock Square in central London. A series of explosions blamed on Islamic terrorists tore into at least three London subway trains and a double-decker bus Thursday, killing dozens and injuring hundreds.

“I didn’t hear anything, just a flash of light, people screaming, no thoughts of what it was. I just had to get out of the train,” said subway passenger Chris Randall, 28, who was hospitalized with cuts and burns to the face, the legs and hands.

“It was chaos,” said Gary Lewis, 32, evacuated from a subway train at King’s Cross station. “The one haunting image was someone whose face was totally black (with soot) and pouring with blood.”

It was the attack that Britain had long feared, following al-Qaida’s Sept. 11, 2001, strikes in New York and Washington and Britain’s subsequent alliance with U.S. forces in Afghanistan and Iraq. Thursday’s explosions also recalled the March 11, 2004, terrorist bombs that killed 191 people on four commuter trains in Madrid, at a time when Spain was part of the U.S.-led coalition in Iraq.

Police were investigating whether suicide bombers were involved, and said they could not confirm the authenticity of a claim of responsibility from a group calling itself “The Secret Organization of al-Qaida in Europe.” The group said the blasts were in retaliation for Britain’s involvement in Iraq and Afghanistan.

In Washington, a senior counterterrorism official said the claim is considered “potentially very credible” because it appeared on a Web site that in the past has been used for extremist postings, the message appeared soon after the attacks and doesn’t appeared hurried or rushed.

The official, who spoke on condition of anonymity because of the sensitivity of the information, also said British authorities identified suspicious packages Thursday and detonated them in controlled explosions. It is not yet clear whether the contents of those packages were dangerous or benign.

The attack on London brought out a steeliness that recalled Britain under the blitz of German bombers in World War II, when many Londoners sought refuge in the Underground, site of Thursday’s carnage.

As Wednesday’s jubilation at winning the Olympics gave way to the terrible shock of Thursday’s attacks, a shaken Blair rushed back to the capital. He then delivered an almost Churchillian appeal for unity, saying in a televised address that it was “a very sad day for the British people, but we will hold true to the British way of life.” He praised the “stoicism and resilience of the British people.”

Both were in evidence across the city, as volunteers helped the walking wounded from blast sites, commuters lent their phones so strangers could call home and thousands faced long lines for homeward-bound buses or even longer walks without complaint.

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“As Brits, we’ll carry on – it doesn’t scare us at all,” said tour guide Michael Cahill, 37. “Look, loads of people are walking down the streets. It’s Great Britain – not called ‘Great’ for nothing.”

Security was raised in the United States and around the world. The Bush administration upped the terror alert a notch to code orange for the nation’s mass transit systems, and bomb-sniffing dogs and armed police patrolled subways and buses in the capital.

The bombings came as Blair and President Bush met over breakfast in Gleneagles, Scotland, and answered questions from reporters, and before all the leaders were due to begin the summit’s general session.

G-8 leaders stood in solidarity with Blair before the prime minister made his hasty departure for London to confer with his Cabinet.

“The war on terror goes on,” Bush said. “I was most impressed by the resolve of all the leaders in the room. Their resolve is as strong as my resolve.”

Based on evidence recovered from the rubble, investigators believe some of the bombs were on timers, a U.S. law enforcement official said.

Investigators doubted that cell phones – used in the Madrid attacks – were used to detonate the bombs in the Underground because the phones often don’t work in the system’s tunnels, the official said. One issue hampering the work is fear that the tunnels themselves may have been damaged in the blasts, the official said, speaking on condition of anonymity because the investigation is ongoing.

An injured subway passenger is escorted away from Edgware Road Tube Station in London after a string of rush-hour explosions Thursday killed dozens.

Among the 700 or so wounded, at least 45 were in serious or critical condition, including amputations, fractures and burns, hospital officials told The Associated Press. Two young women from Knoxville, Tenn., were among those treated for injuries in the Underground, said their father, Dudley Benton.

The first blast caught a subway train between Moorgate and Liverpool Street stations, on the eastern fringe of London’s financial district. Seven died, police said. Moorgate is named for one of the gates in the city walls of London, of which few traces remain. Some people caught in the blast emerged from the Aldgate Station, near Jack the Ripper’s old haunts in Whitechapel.

The second bombing came five minutes later, on a second train deep underground between the King’s Cross and Russell Square stations. Police said 21 died. King’s Cross station, in one of the seediest parts of London, is the film setting for Platform 9 3/4 in the Harry Potter films. Russell Square station serves Bloomsbury, the early 20th-century literary hotbed where Virginia Woolf and luminaries lived.

At 9:17 a.m., there was an explosion involving two or perhaps three trains around Edgware Road station. Seven people were killed, police said. Edgware Road is the heart of a thriving Arab community, and convenient to Hyde Park, scene of last weekend’s Live 8 concert.

The bus explosion, which killed at least two people, took place near Russell Square, an area of many modestly priced hotels popular with tourists. Also nearby is the home where Charles Dickens lived from 1837 to 1839.

Doctors streamed out of the British Medical Association’s offices when the bus blew up outside.

Bystander Raj Mattoo, 35, said the roof of the bus “flew off and went up about 10 meters (about 30 feet). It then floated back down.”

Blair implicated Islamic extremists but cautioned that they speak for only a small percentage of Muslims.

“We know that these people act in the name of Islam,” he said, “but we also know that the vast and overwhelming majority of Muslims here and abroad are decent and law-abiding people who abhor this act of terrorism every bit as much as we do.”

Associated Press writers Mark Sherman and Katherine Shrader in Washington contributed to this report.