The UK's security alert

The UK's "critical" status is one of five security threat levels.

The threat levels are:

Low - meaning an attack is unlikely
Moderate - an attack is possible, but not likely
Substantial - an attack is a strong possibility
Severe - an attack is highly likely
Critical - an attack is expected imminently

The level is set by the Joint Terrorism Analysis Centre (JTAC), which UK authorities created in June 2003.

The last time the level was raised to critical was last August, after police said they had foiled a plot to blow up flights between UK and the US
.

Authorities evacuated the airport after a four-wheel-drive vehicle rammed into building, exploding in flames.

James Edgar, an eyewitness, said the driver of the car fled but was tackled by police.

Susie Cormack, reporting for Al Jazeera at the airport, said the car looked as if it was on fire when it hit a barrier near the main terminal.

The driver then got out of the car "took out what looked like a container of flammable liquid, doused himself in it, set himself on fire and set off to the main terminal and through to the departure hall."

Cormack said the man was unable to make it into the hall as he was stopped by airport security who tried to put the fire out.

Stephen Clarkson, an eyewitness, said: "The jeep is completely on fire and it exploded not long after."

Arrests

Police later said they had arrested two people following the fire at the terminal entrance.

Witnesses described those arrested as Asian men and said one of them was badly burnt. Five bystanders were also treated for injuries.

Later police reports said a "suspect device" was found on a man, who had been arrested with severe burns after the attack and taken to hospital.

The Royal Alexandra Hospital in Paisley, where the man was taken for treatment, was partly evacuated for a time after the device was discovered.

At a news conference, Rae refused to respond to a question about whether the device was a suicide belt, but said: "It was on his person."

"It [the device] was removed and taken to a safe place. It is still the subject of examination," said Rae.

Anti-terrorist police later arrested two more people in the county of Cheshire in north-west England, in connection with the incidents in London and Scotland.

Security alert

Police believe the Glasgow incident is connected
to the bombs found in London [EPA]
Jacqui Smith, the UK's recently appointed interior minister, announced the country's security level had been raised to "critical" after a crisis meeting chaired by Gordon Brown, the UK's new prime minister.

The level suggests the government expects further attacks imminently.

Brown said: "The first duty of the government is the security and safety of all the British people, so it is right to raise the levels of security at airports and in crowded places in the light of the heightened threat.

"I want all British people to be vigilant and want them to support the police and all the authorities in the difficult decisions that they have to make. I know the British people will stand together united, resolute and strong."

The attack on Glasgow airport comes only a day after police discovered two car bombs planted in London.

A vehicle packed with up to 60 litres of fuel, several gas canisters and a large quantity of nails was discovered in London's Haymarket area early on Friday morning.

A second car, also packed with gas and nails, was later found to have been parked close to the first.

In response to the events in the UK, the White House announced that security was being stepped up at US airports, although a spokesman said there was "no indication of any specific or credible threat to the United States."

The developments came exactly a week before the second anniversary of the July 7, 2005 suicide bombings in London which killed 52 people during the morning rush-hour.