Police said an eighth suspect had been arrested in connection with a suspected Al Qaeda plot to detonate car bombs in London and Scotland and police sources said two of those held were doctors.

In the four days since two car bombs loaded with fuel, gas canisters and nails were found in central London, seven suspects have been arrested in Britain, including at least two doctors, and an eighth has been seized in an "undisclosed location".

The BBC reported that he was detained abroad, but that was not immediately confirmed by British investigators.

Of the doctors held, police sources named one as Bilal Abdulla, who qualified in Iraq in 2004, and another as Mohammed Asha, who qualified in Jordan the same year. Asha's wife was also among those arrested.

According to the Muslim News, a Web site that follows the Muslim community, another of those seized in Britain was also a doctor. It quoted a colleague of the man saying he had come to Britain from Bangalore in India.

All those detained in the fast moving investigation are linked to a plot to detonate the car bombs left in central London early on Friday and an attack on Glasgow airport on Saturday using a fuel-laden Jeep Cherokee.

Britain has seen a marked increase in terrorism-related attacks since the September 11 strikes on the United States and since it joined U.S. forces in invading Iraq in 2003.

However, previous assaults, including an attack on London's transport system in July 2005 which killed 52 people, have tended to involve radicalised, British-born Muslims, not educated attackers from overseas, security experts say.

In rapidly progressing operation, police cordoned off a hospital in Paisley, a town just outside Glasgow, on Monday and carried out several controlled detonations.

The hospital, the Royal Alexandra, is where Abdulla worked, staff said, and where he is also believed to be being treated for severe burns after taking part in the attack on Glasgow airport, when his vehicle was turned into a fireball.

Fearing further attacks, police banned cars and other vehicles from directly approaching airports and security measures were stepped up across the country as authorities kept the threat level at "critical", the highest rating.

The series of foiled and actual attacks pose a test for Prime Minister Gordon Brown, who has come under pressure from some quarters to change policy on Iraq and withdraw British troops.

Blair was known for an aggressive stance on security and a foreign policy that strongly supported the United States in Afghanistan and Iraq. The bombers who struck London in 2005 said in videos they were punishing Britain for Blair's policies.

Home Secretary Jacqui Smith said Britain was facing a "serious and sustained threat of terrorism" and urged the public to remain alert. Addressing parliament on Monday, she praised the security services for their quick work in rounding up suspects but said a threat remained.

In Amman, Jordan, the father of Mohammed Asha described his son as a good Muslim, not a fanatic, and expressed incredulity that he could be involved in an al-Qaeda-style bomb plot.

"I am sure Mohammed does not have any links of this nature because his history in Jordan and since he was a kid does not include any kind of activity of this nature," he told Reuters.

He said Mohammed and his wife were happy with their life in Britain and had had a son here about 18 months ago.Reuters