Both bombs seem to have been discovered by accident.  In the first case, an ambulance crew alerted the police after seeing what it thought was smoke inside a silver-green Mercedes parked outside the Tiger Tiger nightclub on the Haymarket. The police defused an explosive device there by hand in the early morning, but did not reveal the episode until hours later.  Then, after a day of growing tensions and reports of a second bomb, the police confirmed Friday night that they had found a blue Mercedes rigged to explode in a car pound on upscale Park Lane, where it was stowed after being ticketed and towed away. Traffic agents said they had smelled gasoline fumes coming from the vehicle.  The car was towed around 3:30 a.m., roughly two and a half hours after the discovery of the first vehicle, the police said. If the cars were supposed to explode in spectacular fashion, the plot had clearly gone awry.  ABC News reported that British security officials said they had seen a “crystal clear” image on a security tape of the driver jumping from the green Mercedes, and that he bore “a close resemblance” to a man arrested in an earlier bomb plot but released for lack of evidence.  A British security official confirmed in an interview on Friday that the authorities were concerned that the supposed attacker might have been a person already known to the authorities who had slipped out of sight after “crossing the radar” in a separate conspiracy.  In recent weeks, several terrorism suspects who were supposed to be restricted in their movements by so-called control orders have disappeared, but they most likely fled abroad, the official said.  The attempted bombings, as well as the potential for further violence, posed an immediate challenge to the newly installed prime minister, Gordon Brown, who convened a meeting of Britain’s top security committee — called Cobra, for Cabinet Office Briefing Room A — to assess the severity of the situation.  “As the police and security services have said on so many occasions, we face a serious and continuous threat to our country,” Mr. Brown said. “But this incident does recall the need for us to be vigilant at all times and the public to be alert at any potential incidents.”  In Washington, counterterrorism officials said that they were following the investigation in London closely, but that they had received no credible reports of possible threats inside the United States, although they urged heightened vigilance with the approach of the Fourth of July holiday.  The Federal Bureau of Investigation issued a bulletin on Friday urging local authorities to step up their watchfulness even though there had been no credible threat reports.  In the course of a jittery day in London, the police barred people and vehicles from Park Lane and urged others to leave adjoining Hyde Park because officers had suspicions, later confirmed, about the car in the underground parking lot.  Fleet Street, leading from central London to the traditional financial district to the east, was also cordoned off, but later reopened. The closings left tracts of central London in gridlock on a busy Friday afternoon when streets are normally packed.  Some Londoners seemed unfazed by the news of the botched attacks. “It’s something you get used to, living in London,” said Andrew Fowler, a 39-year-old lawyer sipping coffee at an outdoor cafe near Piccadilly. “And given the stance our government made on the war in Iraq and elsewhere, I think we are just getting used to being a target.”  The alert, which closed off streets around the Haymarket, brought some people out of nearby offices to find out what was going on.  “It’s only when I got to work that I realized what was happening,” said Renee Anderson, 32, a New Zealander from her country’s nearby diplomatic mission. “I feel surprisingly all right about it. We all kind of thought, ‘Well, you could be hit by a bus anyway.’ ”  News of the developments broke over Britain’s breakfast tables when a police spokesman said explosives experts had discovered a “potentially viable explosive device” in a vehicle. British news organizations quoted witnesses as saying police officers had been seen removing what appeared to be propane gas cylinders and a large number of nails from the car.  Sky News quoted a witness who said the car had been driven erratically before it collided with garbage bins, and that the driver had run off. Mr. Clarke, the counterterrorism official, could not confirm that version of events.  The Tiger Tiger nightclub was packed with hundreds of people at the time the first bomb was discovered. One woman at the club, Rajeshree Patel, told the BBC that the Mercedes had all its doors open and its headlights on. “I think there would have been a lot of fatalities” if the car had exploded, she said. “There were approximately 500 people inside Tiger Tiger at the time.”  The presence of gas cylinders recalled a 2004 terrorist plot called the “Gas Limos Project,” in which Dhiren Barot, a British Muslim accused of being a leading Al Qaeda figure, had planned to use limousines packed with gas cylinders to blow up buildings. In a 39-page planning document, Mr. Barot, who was sentenced in November to a minimum of 40 years in prison, recommended the use of gas cylinders because they were highly destructive and easy to obtain.  In another plot, terrorists were said to have planned to attack the Ministry of Sound, one of London’s biggest nightclubs, using a fertilizer bomb.  Haymarket is in an area of bars, shops and theaters that draws tens of thousands of visitors and revelers. Two theaters on the Haymarket canceled Friday night performances.  The discovery was made one day after the new prime minister, Mr. Brown, formed his first government and close to the second anniversary of the bombings of July 7, 2005, in which four suicide bombers killed 52 people on the London subway system.  The counterterrorism command, headed by Mr. Clarke, has led several major investigations into suspected jihadist conspiracies that have proliferated in Britain since the Sept. 11, 2001, attacks.  In those investigations, suspected terrorists have been accused by the police of planning to use a variety of weapons, including the poison ricin, fertilizer bombs and liquid explosives to attack an array of targets like a shopping mall, a nightclub and airplanes on trans-Atlantic routes.  Jack Straw, who was appointed justice minister by Mr. Brown, said government members had been told about the discovery several hours before the police statement, which was made public as the morning rush hour got under way.  There was no immediate change in the threat level declared by the British authorities. According to the Web site of MI5, the British domestic security service, the current level stands at severe, meaning an attack is “highly likely,” as it has been since August 2006.  Some Londoners said Friday’s alert heightened the sense of suspicion surrounding people of different backgrounds.  With the latest scare, said Sanjay Karsan, 22, a Briton of Indian descent, “I’m worrying that if I walk up that road, they’re going to suspect me.”  Reporting was contributed by Pamela Kent, Beth Gardiner and Raymond Bonner from London, and David Johnston and Mark Mazzetti from Washington.  A version of this article appears in print on , on Page A1 of the New York edition with the headline: London Finds Linked Bombs, A Qaeda Tactic. Order Reprints| Today's Paper|Subscribe
Two Mercedes sedans filled with gasoline, nails and gas canisters had been parked near Piccadilly Circus in the bustling West End.