Britain remained on the country's highest security alert Sunday following three attempted car bombings that have so far resulted in the arrest of five people.

Two men were arrested shortly after a burning sport utility vehicle filled with gas canisters crashed into the front doors of the main terminal building of Glasgow airport on Saturday afternoon.

Police officers are seen in a neighbourhood just west of Glasgow on Sunday as they search for suspects in a firebombing attack at the city's airport.
(Ian Stewart/Associated Press)

Several hours later, a woman and a man who the BBC reported was a doctor were arrested after being stopped in a vehicle on a highway in Cheshire in northern England. A fifth man was taken into custody in Liverpool, police said.

One of the suspects arrested at the airport suffered severe burns and is reported to be in critical condition in hospital. There were reports that police conducted a controlled explosion of a vehicle at the hospital on Sunday.

The bomb squad at Royal Alexandra Hospital did not find explosives, police said.

Elsewhere, officers continued to search a residential area just west of Glasgow for more suspects on Sunday. They also raided buildings in the town of Newcastle-Under-Lyme in central England.

After the attack, Britain raised its security level to "critical" — meaning authorities believe the risk of another attack is imminent. Thousands of vacationers had their travel plans disrupted as the city's airport, Scotland's largest, was sealed off.

Flights were beginning to appear on the departure and arrival boards on Sunday, but passengers were told to expect long delays.

Police said the airport incident was linked to the two car bombs found in the heart of London's entertainment district on Friday. Chief Const. Willie Rae said there were "similarities," and that police were treating the Glasgow attack as a "terrorist incident."

Prime Minister Gordon Brown told the BBC on Sunday it's "clear that we are dealing, in general terms, with people who are associated with al-Qaeda."

He said people still needed to be "constantly vigilant" against what was a "long-term and sustained threat."

"We have to fight it in a number of different ways — militarily, by security, by police, by intelligence," he told the BBC.

On Friday, two Mercedes containing gasoline and nails were found outside a popular nightclub and on a nearby street, just metres from Trafalgar Square and Piccadilly Circus. The devices did not detonate.

Police continue to search for a man seen running from one of the explosives-packed cars.

Extra officers were on guard in Trafalgar Square on Sunday for a mini-marathon.

With files from the Associated Press