British police arrested two more suspects in a hunt for members of a suspected Al-Qaeda cell which rammed a fuel-packed jeep into a Scottish airport and left two car bombs in London, police said on Monday.
Cars and other vehicles were banned from directly approaching airports and security measures were stepped up across the country as authorities kept the threat level at "critical", meaning the possibility of an attack is "imminent".
Police cordoned off housing blocks at a hospital in the town of Paisley near Scotland's biggest city, Glasgow, stoking media speculation that some of those arrested were foreign doctors working in Britain.
The two latest arrests, made on Sunday night in Paisley, took the number detained to seven. Police said the men, aged 25 and 28, were not believed to be of Scottish origin.
A police source said the investigation was going very well and that they expected to make more arrests. The source said the plot bore "all the hallmarks" of Al Qaeda and that there had been no warning of the attack on Glasgow airport on Saturday.
The attacks pose a stiff test for Prime Minister Gordon Brown, who last week replaced Tony Blair. In 2005, Britain was the first country in Western Europe to be hit by Islamist suicide bombers and since then several plots have been foiled.
Blair was known for an aggressive stance on security and a foreign policy which strongly supported the US in Afghanistan and Iraq. The home-grown bombers who struck London transport two years ago, killing 52 commuters, said in videos they were punishing Britain for Blair's policies.
"This continues to be a fast-moving investigation," said Assistant Chief Constable John Malcolm of Strathcylde police in Scotland. "I would continue to urge people to be vigilant." A British security source said it was premature to say whether all those arrested were foreigners. "That's still an area that's being looked at."
Home Secretary (interior minister) Jacqui Smith said Britain was facing a "serious and sustained threat of terrorism" and urged the public to remain on alert. Smith was due to make a statement to parliament on Monday afternoon.
Police declined to identify any of the people under arrest. British newspapers said two were doctors -- one of whom worked at North Staffordshire Hospital in central England.
A spokeswoman at the hospital declined comment. According to Britain's General Medical Council, the man named by media is a registered medical practitioner who qualified in 2004 in Jordan.
On Saturday, police arrested the passenger and badly-burned driver of a Jeep Cherokee who had rammed the vehicle into the entrance of Glasgow's airport, causing a huge fireball.
The attack came 36 hours after police in London defused two Mercedes car bombs packed with fuel canisters, propane tanks and nails, parked in the heart of the capital's bustling theatre and nightclub district.
The other arrests included a 26-year-old man and a 27-year-old woman seized on a motorway in northern England on Saturday, and a 26-year-old man in Liverpool on Sunday. All three were taken to London for questioning and police were given permission to hold them until July 7.
(With input from agencies)