London  - British police Friday foiled two massive car bombings in the heart of London that could have caused “significant loss of life” and placed the city on a high level of alert only three days after the country’s leadership changed hands.

Authorities safely defused both devices planted in the Mercedes vehicles parked not far from each other in one of London’s busiest areas.

British authorities confirmed late Friday that a second vehicle, a blue Mercedes, had been towed after receiving a parking citation near Hyde Park and was then found to contain explosives. Police later inspected the car at the tow yard and safely defused the explosives, said Peter Clarke, head of Scotland Yard’s anti-terrorism unit.

“These vehicles are clearly linked,” Clarke said.

Like the first vehicle, a light green Mercedes found near Piccadilly Circus outside a popular nightclub, the second car was found with a “considerable amount of fuel,” gas canisters and a large quantity of nails, Clarke said. Terrorists frequently use nails in bombs to maximize their killing effect.

British authorities have stepped up security as the country prepares to mark the killing of more than 50 people by suicide bombers who struck London’s transportation system on July 7, 2005.

Prime Minister Gordon Brown took over for Tony Blair on Wednesday and appointed his cabinet Thursday. Brown urged the public to remain vigilant “over the next few days” and warned that Britain remained under “serious and continued threat from terrorism.”

Government sources said the police and intelligence services were investigating a possible international link and similarities to car bombs used by insurgents in Iraq. Home Secretary Jacqui Smith, speaking after an emergency cabinet meeting, said Britain was currently facing “the most serious and sustained threat.”

The counter-intelligence service MI5 warned Friday that further attacks were “highly likely” and that alert levels would remain at “severe,” the second-highest level.

Clarke said after the first bomb was defused that it could have caused “significant injury or loss of life” had it exploded.

“At that hour on a Friday morning, many, many people were leaving nightclubs and other places after an evening out,” Clarke said.

The device had been disabled by officers called to the scene in Haymarket, one of London’s major tourists centres where theatres, clubs, bars and restaurants are located.

Police has earlier cordoned off London’s central Hyde Park to search a suspect car in an underground car park, amid fears that devices could have been planted in other parts of the city.

Brown Friday called an emergency meeting of the government crisis committee.

“I will stress to the cabinet that the vigilance must be maintained over the next few days,” he said, in a reference to the upcoming anniversary of the 2005 attacks that killed 52 people and injured more than 700.

The first suspect car was spotted by an ambulance crew that had been called to the Tiger Tiger nightclub in the Haymarket at about 2 am local time (0100 GMT) and noticed smoke rising out of the Mercedes, Clarke said.

Clarke said it remained unclear if the nightclub was the target of the first car bomb and that he was making no assumptions about who was behind the plot.

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

Among their targets would have been nightclubs with “all those slags dancing around,” according to taped evidence presented to the court.

“The device was almost ready to go, it was primed in a manner to be detonated shortly,” said a BBC correspondent, quoting police sources, about Friday’s attempted attack.

“This was foiled totally by chance,” he added.

An unconfirmed report said the device was to be ignited by mobile phone trigger. Security experts said it was the first time that a so- called vehicle-born improvised device (VBIED) would have been used in an attack in London.

“This is significant. These are the tactics of Iraq coming to the streets of Britain,” said the BBC’s security correspondent, Frank Gardner.

Other experts said the fact that the devices were improvised was also an “al-Qaeda” hallmark.

Brown was informed of the bomb when he woke up, his Home Secretary Smith said. 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.

Sources said there could also be a connection with the expected sentencing next week in the trial of four suspects accused of planning a follow-up attack on subways and buses in London on July 21, 2005.

US and British authorities have been in close contact since the first bomb was discovered and US President George W Bush received a briefing by national security aides at his parents’ oceanfront estate in Maine where on Sunday he will meet with Russian President Vladimir Putin, CNN reported.

US Homeland Security Secretary Chertoff said there was no indication of an imminent attack in the United States, but urged Americans to be vigilant and report anything suspicious as they prepare to celebrate Independence Day on Wednesday.

“Our law enforcement and intelligence officials are closely monitoring the ongoing investigation,” Chertoff said in a statement.