Britain raised its security level to ?critical? ? meaning the risk of terrorist attack is imminent ? following the airport incident, in which a four-wheel-drive vehicle slammed into the main entrance of the terminal and burst into flames.

Two men, one badly burned and in critical condition, were arrested in Glasgow after the attack, which police described as a ?terrorist incident?. Two more people were arrested later on a motorway in northern England.

Wearing white overalls, police in Glasgow searched a number of houses near the airport, in the town of Houston.

?We can confirm that, as part of the ongoing enquiry into the incidents at Glasgow airport and London, a number of houses in the Renfrewshire area are being searched?, police said.

Prime Minister Gordon Brown, who took over from Tony Blair on Wednesday, convened a meeting of Britain?s top security committee to discuss how he would deal with the first big test of his leadership.

Britain?s Home Office (interior ministry) raised the national security alert level to ?critical?, the highest ranking and one which indicates further attacks are expected imminently.

?I want British people to be vigilant and want them to support the police?, Brown said.

The green Jeep Cherokee was driven at speed into the glass door of the airport terminal in Glasgow, 400 miles (600km) north of the capital London, and was then engulfed in flames.

The attack came barely 36 hours after police thwarted a possible al Qaeda plot in London in which two cars loaded with fuel, gas canisters and nails were left in the centre of the capital poised to detonate.

?I can confirm that we believe the incident at Glasgow airport is linked to the events in London yesterday?, the top police officer in the Glasgow area, Willie Rae, told reporters.

?There are clearly similarities and we can confirm that this is being treated as a terrorist incident.?

Rae said the badly burnt man was found to have a ?suspect device? hidden on his body. Reacting to media reports that the device was a suicide belt, Strathclyde Police said on Sunday it was not believed to be an ?improvised explosive device?.

Also on Sunday, London?s Metropolitan Police said they had arrested two people ?in connection with the events in London and Scotland on the 29th and 30th of June?.

A caller to BBC television said he had been in a car in the county of Cheshire, on a motorway which links London with Glasgow, when traffic had been brought to a halt by unmarked police cars. The man, named as Peter Whitehead, said he saw a woman in what he called Muslim dress and two men in suits step out of a car.

The Metropolitan Police said no further details of the arrests were available.

In London, police scoured hours of CCTV footage and extra squads were deployed on the streets after the discovery in the early hours of Friday of the vehicle packed with up to 60 liters of fuel, several gas canisters and a large quantity of nails.

A mobile phone, which security experts believed might have been a detonation device, was left inside the fume-filled car. A second Mercedes packed with gas and nails was later found to have been parked just a few hundred yards from the first.