British police officer stand guard outside King's Cross Underground station that was hit by an explosion, in central London, late Thursday July 7, 2005.(AP)

Terror struck in the heart of London on Thursday as explosions ripped through three subway trains and blasted the roof off a crowded red double-decker bus. At least 37 people were killed and more than 700 wounded in the deadliest attack on the city since the blitz in World War II.

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 and that the blasts at three subway stations went off within 26 minutes, starting at 8:51 a.m. in an Underground train just outside the financial district. 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.

"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.

(Agencies)

blitz : a rapid and violent military attack with intensive aerial bombardment（閃電戰）

soot: a black colloidal substance consisting wholly or principally of amorphous carbon and used to make pigments and ink（煤煙）

retaliation: action taken in return for an injury or offense（報復）