(Releads, adds details on third doctor held, paragraphs 13-14)

By Peter Graff

PAISLEY, Scotland, July 2 (Reuters) - Eight people were being held on Monday in connection with a suspected al Qaeda plot to detonate car bombs in London and Scotland, including at least two doctors detained in Britain.

One of the doctors, named by police sources as Bilal Abdulla, trained and qualified in Iraq in 2004, and another, Mohammed Asha, qualified in Jordan the same year. Asha's wife was among the seven suspects seized in Britain.

An eighth was arrested in an "undisclosed location", the police said, but provided no further details. The BBC reported that he was seized overseas.

All those detained are linked to a plot to detonate two car bombs loaded with fuel, gas canisters and nails in London in the early hours of Friday, and an attack on Glasgow airport in Scotland on Saturday using a fuel-laden Jeep Cherokee.

Britain has seen a marked increase in terrorism-related attacks since the Sept. 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 what authorities have described as a "dynamic investigation", police on Monday cordoned off a hospital in Paisley, a town just outside Glasgow, 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.

A police source said the investigation was going very well and they expected to make more arrests. The source said the plot bore "all the hallmarks" of al Qaeda.

POLITICAL TEST

The series of foiled and actual attacks pose a test for Prime Minister Gordon Brown, a Scotsman who replaced Tony Blair only last week and 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 which 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.

As well as the arrests of Abdulla, Asha and his wife, two more men, aged 25 and 28 and also believed not to be from Britain, were detained in Paisley late on Sunday. Police said they were arrested at the Royal Alexandra hospital complex.

The seventh suspect arrested in Britain was seized in Liverpool earlier on Sunday. The Muslim News, a Web site that covers Britain's Muslim community, quoted colleagues as saying the man was a doctor who came from Bangalore in India.

A British security source said it was premature to say whether all those arrested were foreigners, however. "That's still an area that's being looked at."

ON-GOING THREAT

Home Secretary (interior minister) 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.

(Additional reporting by Peter Griffiths, Michael Holden, Kate Kelland and Mark Trevelyan)

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