LONDON — Police thwarted a devastating terrorist plot yesterday, discovering two Mercedes-Benzes loaded with nails packed around canisters of propane and gasoline set to detonate and kill possibly hundreds in London’s crowded theater and nightclub district.
The plot, coming only two days after Gordon Brown took over as prime minister, recalled the attacks on July 7, 2005, when the London Underground and an iconic double-decker bus were targeted by a group of homegrown suicide bombers who killed 52 persons.
As police searched for car bombs and terrorists in the city of 7.5 million, roads were closed and police sirens wailed. Authorities stepped up security across Britain, from Central London streets to the Wimbledon tennis tournament.
The first car bomb, found near Piccadilly Circus, was powerful enough to have caused “significant injury or loss of life” at a time when hundreds were in the area, British anti-terror police chief Peter Clarke said.
Mr. Clarke said yesterday evening that the second car — another Mercedes — was originally parked illegally on nearby Cockspur Street, but had been towed from the West End to an impound lot near Hyde Park.
“The vehicle was found to contain very similar materials to those that had been found in the first car,” he said. “There was a considerable amount of fuel and gas canisters. As in the first vehicle, there was also a quantity of nails. This, like the first device, was potentially viable.”
There was no immediate claim of responsibility for the bombs.
“We are currently facing the most serious and sustained threat to our security from international terrorism,” Britain’s new home secretary, Jacqui Smith, said after an emergency meeting of top officials.
Police were examining footage from closed-circuit TV cameras, Mr. Clarke said, hoping the surveillance network that covers much of Central London will help them track down the drivers of the cars.
Rep. Peter T. King, New York Republican and the ranking member of the House Homeland Security Committee who was briefed on the investigation, said British authorities had recovered a cell phone that they thought was to be the trigger for the explosion.
“They found a cell phone and it was going to be used to detonate the bomb,” he said.
The U.S. government urged Americans abroad to be vigilant but officials said they saw no potential terrorist threat in the United States ahead of the July Fourth holiday. Department of Homeland Security Secretary Michael Chertoff said there were no immediate plans to raise the U.S. national threat level, currently at yellow, meaning elevated.
New York strengthened its already tight security as a precaution, putting additional police in Times Square and the mass transit system.
The events unfolded when an ambulance crew — responding to a call just before 1:30 a.m. about a person who had fallen at a Haymarket nightclub — noticed smoke coming from a car parked in front of the building, he said.
The crew alerted police, and a bomb squad manually disabled the device, Mr. Clarke said.
Photographs of the metallic green Mercedes first show a canister bearing the words “patio gas,” indicating it was propane, next to the car. The back door was open with blankets spilling out. The car was removed from the scene after a bomb squad disabled the explosives.
Mr. Clarke said there were similarities between yesterday’s incident and an earlier plot, uncovered in 2004, in which an al Qaeda militant planned to detonate gas-fueled bombs in limousines, Reuters news agency reported.
The ringleader of that plot, Dhiren Barot, was convicted last year. Another group of Islamic radicals was convicted this year for a plot in which a big nightclub was among the targets.
There had been no prior intelligence of planned attacks from the al Qaeda terror network, a British government official said on the condition of anonymity because of the sensitivity of the investigation.
The London Daily Telegraph reported that hours before the bombs were found, a message appeared on a Sunni Web site frequently used by al Qaeda and the Taliban that read: “Today I say: rejoice, by Allah, London shall be bombed.”
The message on the al Hesbah chat forum also attacked the recently conferred knighthood for author Salman Rushdie, who faced a death order from Iran’s top ayatollah.
“We, and the whole world has seen what Britain has done. … Their intention to honor Salman Rushdie who insulted and slandered Islam,” the message said.
The Haymarket thoroughfare is packed with restaurants, bars, a cinema complex and West End theaters, and was buzzing at that hour. “Phantom of the Opera” is playing at Her Majesty’s Theater down the street.
It was ladies’ night Thursday, nicknamed “Sugar ’N’ Spice,” at the Tiger Tiger nightclub, a three-story venue that at full capacity can hold 1,770 persons and stays open until 3 a.m.
The discovery of the first bomb triggered a series of security scares across Central London, and police closed Park Lane, Fleet Street and nearby Chancery Lane to investigate other suspicious vehicles.
Mr. Brown, who succeeded Tony Blair as prime minister on Wednesday, called it a reminder that Britain faces a serious and continuous threat of terrorist attacks.
“I will stress to the Cabinet that the vigilance must be maintained over the next few days,” he said.
Londoners seemed relatively unfazed by the news. People crowded onto buses and subway trains during the afternoon rush hour, shopping streets were busy and sidewalk cafes did brisk business.
“Sure, it’s disturbing, and obviously it reminds everyone of 7/7,” said Ian Hiskos, 32, eating at a cafe across the block from the police cordon on Haymarket. “I try not to think about these things.”
The terror threat level in Britain has remained at “severe” — meaning a terrorist attack is highly likely — since last August. But Metropolitan Police sent more officers into the streets of Central London yesterday, and authorities also stepped up security at Wimbledon for the ongoing tennis championship.
One analyst said the bombers could be trying to send Mr. Brown a message.
“It’s a way of testing Gordon Brown,” said Bob Ayers, a security analyst at the Chatham House think tank. “It’s not too far-fetched to assume it was designed to expedite the decision on withdrawal [from Iraq].”
c Staff writer Audrey Hudson in Washington contributed to this article.
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