A series of coordinated rush-hour explosions blasted the transportation system across an arc of central London Thursday, killing at least 38 people and injuring more than 700 people. Authorities expect the death toll to rise.

The explosives detonated on at least three trains moving through London's vast subway system and on a double-decker bus, which had its roof torn off by the blast.

The "coordinated terrorist attack," as police called it, came without warning starting at 8:51 a.m. local time (3:51 a.m. EDT), hurling around some passengers and inflicting severe burns on others. Many suffered smoke inhalation as tunnels and trains filled with thick smoke.

About 45 people were said to have suffered life-threatening injuries.

British Prime Minister Tony Blair, addressing a nation fresh from celebration over being named as the site for the 2012 Summer Olympics, said the attacks were timed to coincide with the G-8 meeting of industrialized nations in Scotland, which President Bush is attending.

" I think we all know what they are trying to do," Blair said. "They are trying to use the slaughter of innocent people to cower us, to frighten us out of doing the things that we want to do, trying to stop us from going about our business as normal, as we're entitled to do.

"And they should not and they must not succeed."

After the attack, the Bush administration raised the terror alert a notch to code orange for the nation's mass transit systems on Thursday. The heightened alert will apply to "regional and inner city passenger rail, subways and metropolitan bus systems," Homeland Security Secretary Michael Chertoff said at a noon news conference.

Several claims of responsibility followed during the day from groups saying they were connected with al Qaeda. None could be authenticated though experts interviewed agreed that the assault bore the hallmarks of an al Qaeda operation.

The explosions were close in timing. According to police, the first occurred at 8:51 a.m. local time (3:51 a.m. EDT) between the Moorgate and Liverpool Street stations of the London Underground on the northern edge of the central city. Seven died in that blast.

The second blast came five minutes later between the King's Cross and Russell Square stations near the British Museum. Twenty-one people died there.

The third explosion detonated at 9:17 a.m. on a train entering the Edgware Road Station, just northwest of Hyde Park. That blast blew a hole in two other trains and claimed five lives.

The fourth, at 9:47, was on a double-decker bus near Tavistock Square in the Bloomsbury area of London. The number of fatalities there remained uncertain. At least three died in the bus blast.

The explosions shut down London's extensive mass transit system, which normally moves about 3 million a day. By the evening rush hour, buses, light rail and train service had started to operate again, according to the Web site for the London Underground. The underground itself was expected to reopen Friday morning

In some respects, the pattern of blasts was similar to that of the attacks in Madrid in 2004 that killed 191 people. Attackers in that assault placed explosive-laden backpacks in trains.

At a news conference at about 4.30 p.m. local time, police said they did not know how the explosives were transported or detonated in London.

Blair left the summit for London after declaring that the leaders gathered there would continue meeting in a show of resistance.

"We will not allow violence to change our values," Blair said, as President Bush and other world leaders looked on grimly. "Nor will we allow it to change our work at the summit. . . . We will defeat those who would impose their fanaticism and extremes on all of us. We shall prevail and they shall not." Blair rejoined the summit shortly after 9 p.m. local time.

Bush directed U.S. security authorities on Thursday to be extra vigilant and take precautions in response to the London attack. He offered a statement of U.S. solidarity with Britain from the summit, which was temporarily suspended as world leaders were briefed on the situation in London.

"The war on terror goes on," Bush told reporters. "I was most impressed by the resolve of all the leaders in the room. Their resolve is as strong as my resolve. And that is, we will not yield to these people, will not yield to the terrorists."

"It does appear to be a coordinated attack on London," Brian Paddick, deputy commissioner of police, said on BBC TV.

London authorities have considerable experience with terrorism and have been on a state of alert for attacks for the better part of four decades. In the past few years, authorities have made numerous arrests of alleged Islamic extremists charging individuals with plotting acts of terrorism in London.

In the '80s and early '90s, the Irish Republican Army staged repeated bomb attacks in Britain and London, causing officials to virtually wall off the financial district of the city, which was a frequent target.

London, and all other large cities, were reminded of the vulnerability of transportation systems when terrorists set off a series of bombs on Madrid trains on March 11, 2004, killing 191 and injuring more than 1800.

London's underground system, inaugurated in 1863, is now bristling with security cameras, which could provide some clues to Thursday's attacks.

After the first reported blast, authorities deployed some troops, bomb squads with dogs and rescue crews across central London.

Hours after the explosions, rescuers were still trying to extricate passengers from some subway carriages in the area of the King's Cross railroad and tube station in the heart of the city.

The police commissioner urged Londoners to "stay where you are" for the time being, saying that authorities were prepared for such situations. He asked for "calm" and pleaded with the media not to speculate.

Emergency vehicles raced through the narrow streets of the city. Some people were seen walking near subway stations covered with blood.

Others were shown on television under blankets.

Witnesses speaking on TV described scenes of flashes of fire, followed by screaming, with passengers on subway cars acting unaided to escape, opening emergency doors and walking through the dark to platforms and up long flights of stairs.

There was "hysteria, pandemonium, smoke," said Angelo Power, a lawyer interviewed on ITN television who was riding on a subway near Russell Square. "Eventually somebody managed to force the door open but nobody would go out because they thought they would be electrocuted. . . . We were trapped like sardines waiting to die and I honestly thought my time was up . . . Finally people started to leave the carriage . . . I saw peoples' belongings scattered all over the place. People were injured. There were no emergency people on hand."

Christine O'Connor, interviewed by the Reuters news agency, was on a Circle Line train at Edgware Road Station. "I was in the last carriage, we pulled out of the station and then almost immediately there was an explosion, it just went very quiet. The carriage filled up with smoke," she told Reuters.

One explosion ripped a red double-decker bus open like a tin of sardines near Russell Square -- a popular area for tourists.

Casualties staggered from the scene, Reuters said.

"I was on the bus. I looked round and the seats behind me were gone," a middle-aged victim said, too shocked and disoriented to say more.

Loyita Worley, 49, was traveling from Moorgate to Aldgate station in the financial district when her Underground train was shaken by a large explosion.

"I saw an orange flickering on the side of the tunnel," she said, adding 20 to 30 walking wounded had been led from the damaged carriage, which had been torn from "floor to ceiling".

"Many were shaking, there were a lot of head injuries, it was very bloody," she said.

One man's clothes had been blown off and he was totally black with soot, she said, but passengers remained calm, even as debris fell down onto the roof of the carriage.

Andrew Allwright, 42, a business development manager at Reuters, narrowly escaped injury at Aldgate East station, Reuters reported.

"As I was walking up the steps there was a loud bang behind me and I felt a blast of air coming from behind. I turned around and saw dust rising from the station," he said.

The swath of the blasts ranges from just northwest of Hyde Park in West London through the area near the British Museum called Russell Square and into the financial district, known as "The City."

The media initially speculated that there had been a power surge of some kind though that was unconfirmed and did not seem to account for reported incidents on buses.

A previously unknown group claimed responsibility in the name of al Qaeda. The "Secret Group of al Qaeda's Jihad in Europe" claimed responsibility for the attack in a Web site posting and warned Italy and Denmark to withdraw their troops from Iraq and Afghanistan, the Italian news agency ANSA and al-Quds al-Arabi newspaper said.

The claim, also sent by e-mail to the London-based daily, could not be verified and did not appear on any of the main Web sites normally used by al Qaeda

Fred Barbash reported from Washington. Washington Post Staff Writers Jim VandeHei in Scotland and Sam Coates in Washington contributed to this story..