Speaking before an emergency meeting of the cabinet's crisis committee, Cobra, Brown said Britain remained under "serious and continuous threat" from terrorism.

He urged the public to stay alert and maintain "vigilance over the next few days," in what was considered a clear reference to the upcoming anniversary of the 2005 attacks, in which 52 people died and more than 700 were injured.

Anti-terrorism officers in forensic suits were seen Friday removing the silver Mercedes car in which the device was found and defused it early Friday morning.

Witnesses said door staff at a night club in Haymarket, in the heart of London's Westend, alerted police after the car was driven into a rubbish bin and the driver ran off.

A witness told Sky television that the large silver car was being driven "erratically" before the minor crash. The driver was not stopped. Another witness reported seeing gas canisters being removed from the car.

Earlier this month, a court in London sentenced seven men, said to have links with al-Qaeda, who had allegedly planned to use explosives-packed limousines for attacks in Britain.

Justice Secretary Jack Straw said the government was informed early Friday of the discovery.

"Sadly, in the world we live in, these things happen," Straw told the BBC in an interview. "The government was told much earlier," added Straw in a comment on the news just made public.

It was revealed later that "enhanced security measures" had been put in place at the Houses of Parliament in Westminster in the wake of the incident.

Earlier, Scotland Yard's anti-terrorism unit launched an investigation following the discovery of a suspected bomb in a parked car.

Officers carried out a "controlled explosion," as parts of the area near Piccadilly Circus were cordoned off, causing major disruption.

London's theatre venues and and the shopping areas of Regent Street and Piccadilly are nearby. The underground (Tube) station at Piccadilly was closed as a result.

Anti-terrorism officers were called to investigate a "suspicious vehicle" at around 2 am (0100 GMT).

There had been no warning, and no arrests had been made.

The size of the "potentially viable explosive device" was not known.

It was unclear whether the device was "being transported" to another location or ready to be exploded where it was found.

The timing of the discovery could be linked to the change of government in Britain this week, commentators said.

Sentencing in the trial of four suspects accused of planning a follow-up attack on tubes and buses in London on July 21, 2005, is expected in the next few days.

Deutsche Presse Agentur