Police in London's bustling 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. Hours later, police confirmed a second car bomb had been found in central London. The second bomb was discovered hours later in another Mercedes that had been illegally parked in the West End but had been towed to an impound lot near Hyde Park, Clarke said at a news conference. He said both contained the same materials: "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." "These vehicles are clearly linked," he said. The discoveries 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. "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. There had been no prior intelligence of planned attacks from the al-Qaeda terror network, a British government official told The Associated Press on condition of anonymity because of the sensitivity of the investigation. Homegrown terrorism is a growing threat in Britain - all four July 7 bombers were Muslims born here - and the planned attack seemed consistent with other homegrown plots, the government official said. 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 driver of the Mercedes. A US legislator briefed on the investigation said British authorities saw a man fiddling with a cell phone near the Mercedes and believe he may have been trying to detonate the device with the phone. "They found a cell phone and it was going to be used to detonate the bomb," Congressman Peter King, the ranking Republican on the House Homeland Security Committee, said in Washington. Sky News television reported that a police officer seized the telephone from the man. 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. A bomb squad manually disabled the device, Clarke said. Photographs of the metallic green Mercedes 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. Hours after the discovery, police closed a major road on the edge of Hyde Park to investigate a suspicious vehicle. Sky News and the British Broadcasting Corp. Reported that the vehicle was linked to the foiled plot, although a police spokeswoman denied any connection had been established. 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 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 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. 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 in New York with gas canisters and shrapnel. 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." 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 were doing brisk business. "Sure, it's disturbing, and obviously it reminds everyone of 7/7," said Ian Hiskos, 32, eating at a cafe near the police cordon on Haymarket. "I try not to think about these things." Since August, the terror threat level in Britain has remained at "severe" - meaning a terrorist attack is highly likely. On Friday, Metropolitan Police said it sent more officers on to 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 prime minister 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)." US officials were in contact with their British counterparts "and will continue to monitor the situation," Gordon Johndroe, the National Security Council spokesman, said in Washington.