LONDON -- Police in London's bustling nightclub and theater district on Friday defused a bomb that could have killed hundreds, after an ambulance crew spotted smoke coming from a Mercedes filled with a lethal mix of gasoline, propane and nails, authorities said.
The bomb near Piccadilly Circus was powerful enough to have caused "significant injury or loss of life" - possibly killing hundreds, British anti-terror police chief Peter Clarke said.
Britain's new home secretary, Jacqui Smith called an emergency meeting of top officials and later said the attempted attack was "international terrorism."
Police were examining footage from closed-circuit TV cameras in the area, Clarke said, hoping the surveillance network that covers much of central London will help them track down the driver of the Mercedes.
Hours after the discovery, police closed a major road on the edge of Hyde Park in response to reports of a suspicious vehicle. Despite the closure of Park Lane, a police spokesman said there was nothing to immediately suggest it was linked to the foiled bombing.
"We are currently facing the most serious and sustained threat to our security from international terrorism," Smith said. "This reinforces the need for the public to remain vigilant to the threat we face at all times."
Officers were called to The Haymarket, just south of Piccadilly Circus, after an ambulance crew - responding to a call just before 1:30 a.m. about a person who had fallen at a nightclub - noticed smoke coming from a car parked in front of the building, Clarke said.
Early photographs of the metallic, pale green Mercedes show a canister bearing the words "patio gas," indicating it was propane gas, 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.
The busy 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 the Her Majesty's Theater down the street.
It was ladies' night Thursday, nicknamed "Sugar 'N' Spice," at the massive Tiger Tiger nightclub, a three-story venue that at full capacity can pack in 1,770 people and stays open until 3 a.m.
The Haymarket venue is Tiger Tiger's flagship club - billed as "stylish and friendly," with live DJs, cocktails and an Asian fusion menu. Owner Novus Leisure also has Tiger Tiger clubs in eight other cities across Britain.
Police were also investigating the possibility that the planned attack could have been criminal in nature.
In response to the foiled bombing, 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 next week's July 4 Independence Day holiday.
"At this time we are characterizing this as a localized incident in London," said Laura Keehner, a spokeswoman for the Department of Homeland Security.
Police in Britain said they did not have any suspects. Authorities urged people who were out in the area to call Britain's anti-terror hot line with any information. Authorities closed the Piccadilly Circus subway station for eight hours and cordoned off a 10-block area around the scene.
Clarke said police would examine footage from the vast network of video cameras in central London equipped with license plate recognition software. There are 160 security cameras in the Westminster Council, the district encompassing Piccadilly Circus and the Haymarket area, alone.
The cameras were put in place following a series of IRA bombing attacks in London in the 1990s - and to enforce London's congestion charge, a toll levied on cars entering central London during certain times of the day.
A British security official, who spoke on condition of anonymity because of the sensitivity of the security details, said there were similarities between the device and vehicle bombs used by insurgents in Iraq.
The official also said the domestic spy agency MI5 would examine possible connections between Friday's bomb attempt and at least two similar foiled plots - to attack a London nightclub in 2004 and to pack limousines with gas canisters and shrapnel.
In the 2004 plot, accused members of an al-Qaida-linked terror cell were convicted of conspiring to cause explosions. One of the possible targets M15 overheard them discussing was the Ministry of Sound, one of London's biggest and most famous nightclubs.
One man is heard saying the plan was to "Blow the whole thing up."
The discovery of the bomb resurrected fears that followed the July 7, 2005, suicide bombings that killed 52 people on three London subways and a bus and failed attacks on the transit system just two weeks later.
Gordon Brown, who only Wednesday succeeded Tony Blair as prime minister, called it a stark 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."
Gordon Johndroe, the National Security Council spokesman, said American officials were in contact with their British counterparts "and will continue to monitor the situation."
The terror threat level in Britain has remained at "severe" - meaning a terrorist attack is highly likely - since last August.
On Friday, Metropolitan Police said it sent more officers on the streets of central London. Authorities also stepped up security at Wimbledon.
One analyst said the bombers could be trying to send Britain's new leader a message.
"It's a way of testing Gordon Brown," said Bob Ayers, a security expert 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)."
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Associated Press writer Raphael G. Satter contributed to this report.
