Two Mercedes loaded with gasoline, gas canisters and nails were found abandoned Friday in what police believe was an attempt to kill scores or even hundreds of people. Detectives said they were keeping an open mind about the perpetrators, but terrorism experts said the signs pointed to an al-Qaeda-linked or inspired cell.

Scotland Yard would not comment on a report by ABC News in the United States that police had a "crystal clear" picture of one suspect from CCTV footage.

Forensics experts were searching the two cars for clues. One was abandoned outside a nightclub in Haymarket, a busy street of shops, clubs and restaurants just yards from Trafalgar Square and Piccadilly Circus.

The other was towed after being parked illegally in nearby Cockspur Street and was discovered in an impound lot about a mile away in Park Lane, near Hyde Park.

Deputy Assistant Commissioner Peter Clarke, the Metropolitan Police anti-terrorism chief, said the two devices could have caused "significant injury or loss of life."

"The discovery of what appears to be a second bomb is obviously troubling, and reinforces the need for the public to be alert," he said.

The plot rattled London a week before the second anniversary of the July 7 suicide bombings that killed 52 commuters on the city's transit system, and two days after Prime Minister Gordon Brown took office.

"London on the Edge" said the front page headline in The Independent newspaper on Saturday, while the Daily Mail asked, "Where's the Next Bomb?"

Brown urged people to be alert, saying Britain faced "a serious and continuous security threat."

The Times newspaper reported that police had distributed a document to nightclubs two weeks ago warning of the threat from "vehicle-borne explosive devices" car bombs. The document, prepared by the National Counterterrorism Security Office, took the form of general counterterrorism advice for British clubs.

The government's crisis committee, code-named COBRA, was due to meet Saturday and police said they were strengthening patrols in the city to reassure the public.

Tens of thousands of people were expected to march through central London later Saturday in the city's annual Gay Pride parade, which ends at Trafalgar Square.

Terrorism experts said the improvised devices discovered Friday were similar to ones used by homegrown terror cells much like the bombs used in the July 7 attacks although the discovery of the second device suggested a coordinated and more sophisticated attack.