LONDON — Commuters in London reluctantly descended into the Underground this morning, attempting to return to routine in the aftermath of four rush-hour blasts that killed at least 37.

"I was scared, but what can you do?" said Raj Varatharaj, 32, emerging from a subway station. "This is the fastest way for me to get to work."

British Home Secretary Charles Clarke today said police hadn't ruled out the possibility that one or more of the blasts was a suicide attack. He told BBC radio that law enforcement's "total effort today is focused on identifying the perpetrators," acknowledging that London could be attacked again.

The worst attack on London since World War II brought out a stoicism that recalled Britain under the blitz of the Nazi Luftwaffe.

"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 — (it's) not called 'Great' for nothing."

Bombs detonated in three crowded subway trains and a double-decker bus during the morning rush hour yesterday, killing at least 37 people and injuring hundreds of others.

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 blast from the sidewalk. "About four or five people literally came flying out the top."

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 world powers, who had gathered in Gleneagles, Scotland, with other heads of government.

A previously unknown group calling itself the Secret Organization of Al Qaida in Europe claimed responsibility for the bombings in a letter posted on a Web site used by extremists.

"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. There was no confirmation of whether the statement was authentic.

Investigators were looking into the possibility that a terrorist was killed aboard the bus, but they were unsure as to 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 wars in 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 Blair spoke, 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 afterward looking grim and shaken to condemn this "terrible and tragic atrocity" and 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. ... We will not be terrorized," said Blair.

JUBILATION SMASHED

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 jubilation was smashed yesterday morning in attacks that 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, a few miles to the west. And 21 minutes after that, a third bomb ripped open a train pulling into the Edgware Road station further 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 wounded.

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

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."

'IT WAS TERRIFYING'

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 Place in the city's Bloomsbury section.

"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 doctor who treated victims at the scene, said he was told at least 10 people had died, and said several other victims had suffered life-threatening injuries. Several of the most badly injured had been pedestrians on the sidewalk rather than passengers, he said.