Bombs detonated in three crowded subway trains and on a double-decker bus during the morning rush hour here Thursday, killing at least 37 people and injuring about 700 others in the deadliest terrorist attack carried out on British soil.

The first three blasts devastated the trains in quick succession, hurling broken metal, glass shards and body parts in every direction. Survivors fled down dark tunnels to escape flames and thick black smoke.

Thirty minutes later, a fourth explosion tore the top off a crowded red London-style bus and rained blood and bodies on the pavement below. "One moment there was a bus there, and the next moment it peeled up like a top of sardines," said Billy Palmer, 42, a musician who witnessed the explosion from the sidewalk. "About four or five people literally came flying out the top."

No arrests were announced, but British officials immediately pointed to Islamic extremists as the most likely perpetrators, citing the coordinated nature of the bombings and the timing -- on the first full day of a summit of leaders of the Group of Eight industrial nations, who had gathered in Gleneagles, Scotland, with other heads of government.

A previously unknown group calling itself the Secret Organization of al Qaeda in Europe asserted responsibility for the bombings in a letter posted on a Web site used by extremists. The claim could not be verified.

"Rejoice for it is time to take revenge against the British Zionist Crusader government in retaliation for the massacres Britain is committing in Iraq and Afghanistan," the statement said.

Investigators were looking into the possibility that an attacker was killed aboard the bus, but they were unsure whether the bomb might have detonated accidentally while being transported or was set off in a suicide attack. The train bombings, in contrast, appeared to have been carried out by attackers who placed explosives in the trains and left.

British Prime Minister Tony Blair, who has been President Bush's closest ally in the invasions of Iraq and Afghanistan, interrupted the summit to make a statement of anguish and outrage before television cameras. He declared that the bombers "will never succeed in destroying what we hold dear in this country and in other civilized nations."

As he spoke, President Bush and the leaders of China, Japan, India, Germany, France, Italy, Russia, South Africa, Brazil, Mexico and Canada stood behind him in a show of solidarity. U.N. Secretary General Kofi Annan was also present.

Blair then flew back to London to chair a special session of cabinet ministers, security officials and emergency coordinators. He emerged looking grim and shaken to condemn this "terrible and tragic atrocity" and to pledge an intense effort to capture those responsible.

"It's through terrorism that the people that have committed this terrible act express their values, and it's right at this moment that we demonstrate ours," said Blair, who added, "We will not be terrorized."

Less than 24 hours earlier, Londoners had literally danced in the streets to celebrate the city's surprise triumph over Paris in winning the right to host the 2012 Olympic Games. But the mood of jubilation was shattered Thursday morning by the attacks, which security police here had long predicted as inevitable but for which they said there had been no warning.

The first explosion occurred at 8:51 a.m. in the front car of a train in a tunnel between the Liverpool Street and Aldgate stations near the heart of London's financial district, known as the City.

Five minutes later, a second blast tore through a train between the King's Cross and Russell Square stations, about two miles to the west. And 21 minutes after that, a third bomb ripped open a train pulling into the Edgware Road station farther to the west, where it collided with two other trains. Witnesses said a huge hole opened in the floor of one car and that one man was sucked out through the opening.

John Simpson, a banker from Gloucester, who was on his way to Aldgate on the first train that was hit, recalled being blinded by a shining light before the blast hit him. "It shook me up and I thought I was on fire," he said after his release from a hospital. "I tried to put myself out. There were a couple of people with head injuries in my carriage. One of the drivers of another train came out and let us all out the back door. There was quite a few people lying on the track. I would say they were dead."

Officials said that preliminary counts showed the first and third blasts killing seven people each and the second causing 21 fatalities. Officials warned that the toll would probably rise overnight. Hundreds more people were injured.

Ever since the train bombings in Madrid that killed 191 people in March 2004, the emergency services here have redoubled their preparations for a similar attack, and on Thursday they quickly launched a preset operation.

Officials rushed more than 100 ambulances and other emergency vehicles to the three scenes, ferried survivors to safety above ground and then rushed the injured to one of a half-dozen designated hospitals. But some witnesses said rescue operations were delayed for critical minutes at some sites because officials feared further blasts.

Gareth Davies, medical director of the London Air Ambulance, said he got to the Aldgate site at 9:10 a.m. "There were lots of walking wounded outside the station," he said. "There were others in the foyer. The more severely injured were on the track and in the wreckage. It was quite overwhelming, it shocked everybody who was in the area, but there was efficiency -- there wasn't much noise."

The authorities shut down all London Underground subway trains, which normally carry 3 million people daily. But they allowed buses to continue running.

Then, exactly 30 minutes after the Edgware Road attack, a fourth bomb went off in the upper section of a packed double-decker bus in Tavistock Square, in the city's Bloomsbury district.

"Everybody ran for cover in a shop doorway," Sandra Pollins, who was on the street near the bus, told ITV News channel. "It was terrifying. It took a minute or two to compose ourselves, then we came out. I could not even recognize that it was a bus. The whole roof had been blown off. There were people just walking around with blood all over their faces."

Officials early on put the death toll there at two, but Laurence Buckman, a physician who treated victims at the scene, said he was told that at least 10 people had died and that several others had suffered life-threatening injuries. Some of the most badly injured had been pedestrians rather than passengers, he said.

The bus driver, who was shaken but uninjured by the blast, told Buckman that the explosion occurred near the rear of the vehicle's upper deck. A suicide bomber could probably have caused more casualties by detonating the bomb on the lower deck. An alternative theory was that the bomb went off by accident while being carried by an attacker. Police declined to comment on the placement of the bomb.

After the fourth explosion, all bus service was suspended in the city center and people were advised to stay home and avoid traveling. Police sealed off the four blast sites and investigators began examining the rubble. Police also cordoned off areas around Buckingham Palace, Parliament and Blair's Downing Street home and office.

Alastair Wilson, clinical director of the Royal London Hospital, said that 183 patients were admitted to the accident and emergency department and 123 had been discharged. Of those still in the facility, eight were critically injured, one of whom had arrived during a cardiac arrest. Six people were currently in the operating theater.

"The patients are suffering from injuries including blast injuries, limb injuries, inhalation injuries from smoke," he said. "People are having problems with their hearing as a result of the blast and problems with their eyes which will go on for a while but will stabilize. The closer people were to the blast, the more serious their injuries. I am sure there will be more fatalities from the scene."

By late morning, many streets were deserted and a restless silence descended on the city, shattered periodically by ambulance sirens or the whirring of police helicopters overhead. Afternoon rush hour was in full swing by 4 p.m., but without subways and buses, most workers had to walk.

In scenes reminiscent of Sept. 11, 2001, in New York, Londoners undid their ties, took off their jackets and consulted maps to find their way on foot through their own city.

Officials announced that parts of the Underground would remain closed Friday for repair work and investigation, while buses, above-ground commuter rail service and the rest of the subway would resume operation. Schools would be closed.

Charles Clarke, the cabinet secretary in charge of domestic security, urged Londoners to continue living as normally as possible. "The aim of the terrorists is to try and stop us leading our lives as we best can," Clarke told the BBC, "and I think our responsibility is to try and get on and live our lives as best we can."

News of the bombings led to a plunge in world financial markets, with European exchanges bearing the brunt of the damage, but they recovered steadily throughout the day.

Correspondent Ellen Knickmeyer and special correspondent Audrey Gillan contributed to this report.

A victim is helped from the area of the Russell Square subway station, near where the second of the bombs exploded. At least 21 people were killed in that blast. Overall, about 700 people were injured in the four bombings in less than an hour. With the Houses of Parliament in the background, pedestrians walk across Westminster Bridge after London's public transportation system was shut down in the wake of bombings on subway trains and a bus. An injured person is assisted outside the Edgware Road subway station, where at least seven were killed in the third bombing. "People are having problems with their hearing as a result of the blast and problems with their eyes which will go on for a while but will stabilize," a hospital official said.