Three terrorist bombs ripped through London Underground trains during the morning rush hour yesterday and a fourth destroyed a double-decker bus, killing at least 37 people and wounding more than 700 others.

The tube, or subway, was shut down and bus services in the center of the city halted, causing widespread travel disruption. Mainline stations were also closed for much of the day.

Last night, the foreign secretary, Jack Straw, said the explosions “bore all the hallmarks” of Al Qaeda. The coordinated attacks were being compared to the train bombings that killed 190 people in Madrid last year.

There were fears that the bus explosion, which left two people dead in Tavistock Square, near the British Museum, was the work of a suicide bomber. If so, it would be the first such incident in Britain and Europe, indicating that vigilance by transport staff and passengers could be no defense against outrages.

[Based on evidence recovered from the rubble, investigators believe some of the bombs were on timers, an American law enforcement official said on condition of anonymity because the investigation was ongoing, the Associated Press reported.

[Investigators doubted that cell phones – used in the Madrid train attacks last year – were used to detonate the bombs yesterday because reception is spotty in the Underground’s tunnels, the official said.

[The senior American counterterrorism official said there was no confirmation “at all” so far that suicide bombers were involved.

[The official said British authorities identified suspicious packages yesterday and detonated them in controlled explosions. It is not yet clear whether the contents of those packages were dangerous.]

The attacks were the worst terrorist attack on British soil after the bombing of Flight 103 over Lockerbie in 1988, which killed 270 people.

The queen, “deeply shocked,” sent a message to victims and relatives, and Prime Minister Blair urged defiance to the bombers who were trying to “cow” the nation.

Looking shaken by the extent of the carnage, Mr. Blair said: “The purpose of terrorism is just that; it is to terrorize people – and we will not be terrorized. This is a very sad day for the British people, but we will hold true to the British way of life.” He said “this most terrible and tragic atrocity” had cost many innocent lives. Mr. Blair promised “the most intense police and security service action to make sure that we bring those responsible to justice.”

He added: “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.”

The prime minister had flown back to London from Gleneagles where he had been hosting the meeting of G-8 leaders and brought with him promises of resolute support from countries as varied as America, Mexico, Russia, and China, all of which have had their own experiences of terrorism.

President Bush said: “It’s a war on terror for us all.”

The first bomb exploded at 8:51 a.m. in a train on the Circle line as it traveled from Aldgate station, in east London, to Liverpool Street.

The train was more than usually packed because of an earlier disruption caused by signaling problems. Seven people were killed and dozens hurt.

Underground staff had just registered the emergency calls flooding along their internal communication lines when, five minutes later, another bomb went off.

This time it was on a Piccadilly line train traveling west from King’s Cross to Russell Square. The carriages were crammed with commuters and 21 were killed.

Within minutes, ambulances and police began to arrive in force, following a detailed emergency plan that service leaders had developed in case of such an attack.

Survivors and the walking wounded were being led to safety 300 yards back down the track to King’s Cross and ambulance personnel and fire crews were hurrying past them towards the dead and the maimed when radio circuits gave news of still further carnage at 9:17 a.m.

This time a bomb shattered a westbound Circle line train as it was leaving Edgware Road station toward Paddington. The blast tore through a wall and caused damage to two other trains. Seven people were killed and dozens of others injured.

Staff from a hotel helped medical teams from St. Mary’s Hospital, Paddington, to care for the walking wounded.

The fourth bomb, in a red doubledecker passing the headquarters of the British Medical Association, went off at 9:47 a.m. The blast ripped off its roof.

The bus, a no. 30 from Hackney Wick to Marble Arch, had been rerouted because of the disruption caused by the attacks on the Underground.

The injured were tended by passersby, including an American who had taken a first-aid course, Stephanie Riak Akuei. She helped bandage and comfort those trapped on the bus.

She said: “I was on the corner and heard the noise. I went over and there were at least seven people who were obviously dead. There was a lot of body parts and human debris.

“There were people dead, people alive, trapped on the bus.” She was joined by doctors from the BMA building and later by paramedics.

The capital, which a day earlier had been celebrating its unexpected victory in the race for the 2012 Olympics, was given a warning by police that the explosions could be part of a campaign of terrorism. They said that thousands of officers would be deployed across the transport network today.

The deputy chief constable of British Transport Police, Andy Trotter, said: “We don’t know whether this is over.”

No warning of the blasts was given and the intelligence services said that they had no indication from any sources that an attack was imminent.

MI5 and Scotland Yard’s anti-terrorist branch have consistently said that an attack on London, or anywhere else in the country, was inevitable and that the only question was when it would happen.

Police said that at one stage they were receiving 42,000 calls an hour from anxious relatives and were asking members of the public to try to make direct contact with them first.

With mainline stations closed for much of the day and the Underground for all of it, people who had managed to reach work were faced with a difficult journey home.

Today many firms were arranging shuttle buses and taxis to get people to their offices.

At King’s Cross, Keith Hill, 36, a manager at the Football Association heading home said: “It might sound like a cliche, but this could be the safest time to take a train.”

As some railway stations reopened, thousands of people leaving the city for suburban homes began to wonder if their commuter mornings would ever be the same again.

The queen ordered the Union flag over Buckingham Palace to fly at half-mast, and flags over all government buildings will also be lowered. In a statement from the palace, the queen said: “The dreadful events in London this morning have deeply shocked us all.

“I know I speak for the whole nation in expressing my sympathy to all those affected and the relatives of the killed and injured. I have nothing but admiration for the emergency services as they go about their work.”

Michael Howard said it was a “dreadful day for London and for our country.”

“I entirely support what the Prime Minister has said about our determination to protect and defend our way of life.

“It is important that everyone should know that this country is completely united in our determination to defeat terrorism and to deal with those who are responsible for the appalling acts which we have seen today.”