LONDON – Police in London’s bustling nightclub and theater district on Friday defused a car 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. Hours later, police confirmed a second explosives-rigged car was found nearby.

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.

Clarke said Friday 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.”

The discoveries came just ahead of the second anniversary of the July 7, 2005, suicide bombings that killed 52 people on three London subways and a bus.

“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.

In Washington, two officials said British authorities found no link between the defused car bomb and any terrorist group during the early hours of the investigation. The officials, who were briefed on the inquiry, said the investigation had yielded no suspects and no definitive description of anyone leaving the vehicle. The officials spoke on condition of anonymity because they were not authorized to discuss the case.

Police were examining footage from closed-circuit TV cameras, Clarke said, hoping the surveillance network that covers much of central London will help them track down the drivers of the cars.

Rep. Peter King, R-N.Y., 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 believed 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 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, Clarke said.

The crew alerted police, and a bomb squad manually disabled the device, Clarke said.

Photographs of the metallic green Mercedes discovered 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.

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 pack in 1,770 people and stays open until 3 a.m.

Police also were investigating the possibility that the planned attack could have been criminal in nature. Authorities closed the Piccadilly Circus subway station for eight hours and cordoned off a 10-block area around the scene.

The incident 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.

Gordon Brown, who only Wednesday succeeded Tony Blair as prime minister, 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.”

There had been no prior intelligence of planned attacks from the al-Qaida terror network, a British government official told The Associated Press on condition of anonymity because of the sensitivity of the investigation.

Londoners were 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.

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).”

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.

New York strengthened its already tight security as a precaution, putting additional police in Times Square and the mass transit system.

“We’re going to ramp up a little bit, but nothing dramatic,” Mayor Michael Bloomberg said on his weekly radio show. “We’ll take a little bit of extra precaution. Some of you will notice, some of you won’t – but we have to be cognizant.”

Associated Press writers Raphael G. Satter in London and Matt Apuzzo and Katherine Shrader in Washington contributed to this report.