LONDON – Bomb blasts on underground trains and a double-decker bus during this city’s Thursday morning rush hour killed at least 37 people and wounded 700 in a terrorist attack that unsettled governments and people on both sides of the Atlantic.

A previously unknown group, calling itself the “Secret Organization of al-Qaida in Europe,” claimed responsibility for the coordinated bombings and said Britain is “now burning with fear and terror.”

Within hours, the Bush administration raised the terror alert level for mass transit systems in the United States, although officials said they had no information that an attack was planned on American soil.

The blasts left London residents stunned just one day after jubilant crowds had partied late into the night to celebrate the city winning the right to host the 2012 summer Olympic Games.

With its subway and bus systems closed, London came to an eerie standstill. Stores also closed, and masses of stranded commuters left downtown on foot. Many people said they planned to walk miles home, in the absence of public transport.

Grim-faced, British Prime Minister Tony Blair rushed back to London from the Group of Eight summit in Gleneagles, Scotland. He blamed Muslim terrorists for the attack.

“We know that these people act in the name of Islam,” he said. He promised the “most intense police and security service action to make sure we bring those responsible to justice.”

Muslim leaders in Britain quickly condemned the bombings and said they feared that vigilantes might target Muslims for revenge.

“These terrorists, these evil people want to demoralize us as a nation and divide us,” Iqbal Sacranie of the Muslim Council of Britain told the British Broadcasting Corp.

There was no official word on whether the bombs had been planted or were triggered by suicide attackers, but U.S. government officials said that there was growing consensus that all or most of the bombs involved devices that were presumably timed or remote-controlled.

A U.S. senior intelligence official, speaking on condition of anonymity because of the sensitivity of the issue, said late Thursday that British officials found some suspicious items after the blasts, including one that they detonated in a controlled explosion. There was no indication of what those items were.

Many people expressed fear that other attacks were imminent.

The British pound weakened, and Britain’s main stock index, the FTSE 100, plunged 4 percent before recovering slightly to close down 1.42 percent. Stocks in other European capitals also slumped.

London’s West End theater district, a major tourism draw, went dark for the evening, and hotels bulged with stranded workers unable to get home and looking for lodging.

Concern spread well beyond London. In Washington, police wearing military fatigues and bulletproof vests and toting machine guns rode the subway. In San Francisco, civilian transit workers donned iridescent green vests and joined police on patrol. Boston’s transit system will run empty trains after the subway system normally shuts down to deter “any activity where there should be none,” a transit official said.

“Obviously, we’re concerned about the possibility of a copycat attack,” Homeland Security Secretary Michael Chertoff said at a news conference in Washington. The department planned to send out an advisory bulletin Thursday night to federal, state and local law enforcement agencies warning that U.S. mass transit systems, public gathering places and aircraft could still be potential targets.

Similar precautions were taken throughout Europe. Italy raised alert levels at its airports, and security was beefed up at airports, railroad stations and subways in the Czech Republic, Hungary, Russia, the Netherlands, France and Spain.

The blasts were the largest attack on London since World War II and the first major terror action in Europe since coordinated bombings on four Madrid trains last year killed 191 people and wounded nearly 2,000.

The attacks came on a drizzly, somewhat chilly morning, beginning at 8:48 a.m. and ending at 9:45 a.m. local time. One of the bombs tore off the roof of a double-decker bus next to Tavistock Square near the University of London. Three others ripped through packed tube trains near the Liverpool Street, King’s Cross and Edgware Road stations.

“People were physically ejected out of their chairs. … The hysteria almost became pandemonium,” one train survivor told ITV television.

At least 21 people died in the blast at King’s Cross, said Brian Paddick, deputy assistant commissioner of the Metropolitan Police.

Paddick said seven people were killed on a train near the Liverpool Street station, and seven died at Edgware Road. Two were killed on the double-decker bus, Paddick said.

Authorities halted all subway traffic, initially declaring that there’d been an electrical outage. But newscasts quickly informed the public of the explosions.

Other politicians joined Blair, who called the bombings “barbaric,” in decrying the attacks.

At the peaceful, lush green setting of the Gleneagles resort hotel in Scotland, where world leaders were discussing the global economy, aid to Africa and climate change, a solemn President Bush expressed disgust at the London attacks.

“The war on terror goes on,” Bush said. “… We will not yield to these people; we will not yield to the terrorists. We will find them, bring them to justice, and at the same time, we will spread an ideology of hope and compassion that will overwhelm their ideology of hate.”

Bush spoke after conducting a 10-minute video conference call with his homeland security and national security advisers from his suite.

Still, one terror expert said the impact on London would be serious.

“The effects are still the same. They’ve managed to overshadow the G-8 summit. And there’ll be longstanding effects on the British economy, particularly in tourism,” said Magnus Ranstorp, a terrorism expert at the University of St. Andrews in Scotland.

Transport authorities said the underground train system would operate only partially on Friday as officials hunt for clues to prosecute those behind the attacks. British officials are hoping to get clues to the attackers’ identities from the hundreds of remote cameras that have been installed around London.

Ally of Bush

Blair’s government has been a staunch ally of the Bush administration as it has mounted military attacks on Afghanistan and Iraq in the war on terrorism. Blair has sent British troops to Iraq despite Britons’ largely opposing the deployment.

Ranstorp said Britain’s counterterrorism agency had mounted a “robust” campaign to shut down radical terror cells and track individuals, but that bombings like those on Thursday are difficult to prevent.

“Other European capitals should be worried that this will come to them,” Ranstorp said.

Amid the near constant wail of sirens, Londoners praised emergency rescuers and police.

But David Davis, a Conservative member of Parliament who serves as a shadow home secretary, said the bombings appeared to have caught security services by surprise.

“It looks as though we didn’t detect the attack at all in advance,” Davis said.

In Washington, Chertoff personally announced the heightened alert for mass transit systems. But there appeared to be little of the political sniping that characterized the Homeland Security Department’s decision last summer to raise the terror alert for financial institutions in New York City; Newark, N.J.; San Francisco and Washington. The department then was accused of playing politics after it was revealed that the higher alert was based on documents and intelligence that were more than a year old.

Washington, D.C., officials said Thursday that federal officials have kept them informed about the heightened security measures and that they’d received notification about the heightened alert on transit systems before it was announced publicly.

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