London Police, defused an explosive car loaded with gasoline, gas canisters, nails and a detonator after an ambulance crew spotted smoke coming from the vehicle.
The explosives were 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, calling the attempted attack "international terrorism."
"We are currently facing the most serious and sustained threat to our security from international terrorism," she told reporters. "This reinforces the need for the public to remain vigilant to the threat we face at all times."
Police planned to examine 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 rigged Mercedes.
Officers were called to Haymarket, just south of Piccadilly Circus, after an ambulance crew responding to a call just before 1:30 a.m. (0030GMT) about an injury at a nearby nightclub noticed smoke coming from a car parked in front of the club, Clarke said.
A bomb squad was called to the scene, and manually disabled the bomb.
Early photographs of the silver Mercedes showed 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 midmorning.
The busy Haymarket thoroughfare linking Piccadilly Circus to the Pall Mall is packed with restaurants, bars, a cinema complex and West End theaters, and was buzzing at that hour.
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; owner Novus Leisure also has clubs in other cities across Britain.
Police said they did not have any suspects, and 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 so-called "ring of steel" a network of video cameras equipped with license plate recognition software.
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.
The British security official 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 plotting to blow up the Ministry of Sound nightclub. A recording made by MI5 captured the plotters discussing an attack on the nightclub, one of London's biggest and most famous venues.
One man is heard saying the plan was to "Blow the whole thing up."
Friday's attempted bombing comes just two days after Gordon Brown succeeded Tony Blair as prime minister, and a week before the second anniversary of the deadly July 7 London bombings that killed 52 people and the four bombers.
Brown called it a stark reminder that Britain faces a serious and continuous threat of terrorist attacks. He urged people to be on alert.
"I will stress to the Cabinet that the vigilance must be maintained over the next few days," he said.
The terror threat level has remained at "severe" meaning a terrorist attack is highly likely since last August.
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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