London is on alert - police have stepped up patrols and a search is under way for a man seen running from a Mercedes car that contained gas canisters, gasoline, nails and a cell phone that was believed to be the detonating device.

The Mercedes was abandoned in the Haymarket district of London - an area full of nightclubs, restaurants and theaters. It raised suspicion when ambulance drivers called to a nearby nightclub about an accident noticed smoke coming from the Mercedes and called in police. There are unconfirmed reports one of the policemen found the cell phone and removed it from the vehicle - thus averting an explosion.

A second car was parked illegally nearby and was towed to an impound lot, where police later discovered similar explosive materials.

The head of London's anti-terrorism police, Peter Clark, said the two incidents are clearly linked, and he said had they not been found in time, the car bombs could have killed hundreds of people.

London Mayor Ken Livingstone has tried to reassure residents.

"This is what's great about the way Londoners do react to these things, they put them in perspective... I think people will be completely safe to walk about the streets of London today," he said.

Police have so far not named any suspects. However, terrorism experts say indications are that individuals or a group either linked to or inspired by al-Qaida was behind the bombing attempts.

Next week, Britain marks the second anniversary of the July 7 bombings, in which four Muslim suicide bombers set off explosions on London's transport system that killed 52 bus and subway passengers.

Friday's attempted bombings came just two days after Gordon Brown took over from Tony Blair as Britain's new prime minister. Mr. Brown called for heightened vigilance on part of the entire population amidst this latest threat.