British police stepped up their patrols in London on Saturday after the discovery of two car bombs packed with petrol, gas and nails in the capital’s teeming theatre district.

Authorities launched a counter-terrorism investigation after Friday’s discovery of explosives in two parked Mercedes cars in London’s West End that had echoes of an earlier al Qaeda plot.

With high profile events scheduled in London at the weekend including a Gay Pride march, the Wimbledon tennis championships and a concert in honour of Princess Diana, police maintained a high alert.

“There will be more police patrols. The investigation is moving ahead,” anti-terrorism police chief Peter Clarke said.

Tens of thousands of people were expected to attend the Gay Pride march which will follow a route from Baker Street to Trafalgar Square on Saturday. Wimbledon tennis would also continue following a review of security arrangements, police superintendent Peter Dobson said.

Detectives were checking CCTV footage from the area around Haymarket and Cockspur Street where the two cars were discovered and carrying out forensic tests. Clarke said the bombs in both the cars contained gas tanks, fuel canisters and nails. He added there were similarities between Friday’s incident and an earlier plot, uncovered in 2004, in which an al Qaeda militant planned to detonate gas-fuelled bombs in limousines.

The ringleader of that plot, Dhiren Barot, was convicted last year.

Meanwhile, Prime Minister Gordon Brown has convened Britain’s top security committee, Cobra.

“We are currently facing the most serious and sustained threat to our security from international terrorism,” Jacqui Smith, Brown's new Home Secretary, said after the meeting, which she chaired in her first day on the job.
Sky News said the first device was rigged to detonate with a mobile-phone trigger. Police would not confirm that report.

Intelligence sources said they could not rule out an al Qaeda link.

 A security source said: “The balance of probability does lie pretty strongly with international terrorism.”