The UK's national terrorism threat has been raised to its highest level after an attack on Glasgow Airport was linked to two car bombs found in London.
Two men were arrested after ramming a burning car into the airport's main terminal, a day after police thwarted two attempted bombings in the capital.
Police said one of the men wore a "suspect device" and the incident was being treated as terror related.
Prime Minister Gordon Brown urged the public to remain vigilant.
"I know the British people will stand together, united and resolute," he added.
Strathclyde Police told a news conference the attack on Glasgow Airport was now being linked to the car-bomb plot in London - foiled by police on Friday.
Chief Constable Willie Rae said: "There are clearly similarities, and we can confirm that this is being treated as a terrorist incident."
Two men were arrested at the scene at 1515 BST after the blazing car was driven into the airport's main terminal.
Mr Brown thanked the emergency services for their professionalism
One of the suspects taken to the city's Royal Alexandra Hospital to be treated for severe burns was in possession of "a suspect device" which had been found on his person, Chief Constable Rae said.
BBC security correspondent Frank Gardner said the suspect device was thought to be a "suicide belt".
The second man was being detained in police custody while the vehicle was said to be too unstable to be removed from the airport.
Eyewitnesses described a Jeep Cherokee being driven at speed towards the main doors of Glasgow airport's terminal building with flames coming out from underneath shortly after 1500 BST.
All flights to and from Glasgow Airport have been suspended.
A number of other airports have stepped up security, including Edinburgh, Newcastle, Birmingham, Manchester and Blackpool. John Lennon Airport in Liverpool has been closed as a precaution.
Security has also been tightened at airports across the US following the attempted attacks.
The national terrorism threat level was raised after a meeting of the government emergencies committee, Cobra.
Ministers, police and security service officials held their third meeting of Cobra in recent days on Saturday evening, this time with a video link to ministers in Scotland.
TERROR THREAT LEVELS
Low - an attack is unlikely
Moderate - an attack is possible but not likely
Substantial - strong possibility of an attack
Severe - an attack is highly likely
Critical - an attack is expected imminently
In a televised statement, Mr Brown thanked the police, the security and emergency services for the "dedicated professionalism" that has been shown in London and Glasgow Airport.
"The first duty of the government is the security and safety of all the British people, so it is right to raise the level of security at airports and in crowded places in the light of the heightened threat," he said.
The critical threat level indicates terrorist attacks are imminent. It is the highest possible level and was also in place after the 7 July 2005 suicide bomb attacks on London.
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A Scottish government spokesperson said Scottish Ministers were in touch with their Whitehall opposite numbers and First Minister Alex Salmond and have spoken to Mr Brown.
Mr Salmond and Justice Secretary Kenny MacAskill took part in the Cobra meeting.
Mr Salmond echoed the prime minister's calls for vigilance.
"We cannot and must not live our lives in a state of alarm but alert individuals have a substantial role to play in supporting our police services," he added.
Flights from Glasgow airport were suspended after the crash
Earlier in the day, Cobra met to discuss the London car bomb attempts.
In the early hours of Friday, two Mercedes containing petrol, gas cylinders and nails were found left outside the Tiger Tiger club in Haymarket and a nearby street but the devices did not detonate.
Police in the capital are checking CCTV footage in their investigation into the planting of the two car bombs.
Unconfirmed reports suggest police may have an image of a suspect leaving the vehicle left outside the Tiger Tiger club.
Police increased patrols and security for events in London over the weekend, including the Gay Pride parade, the Concert for Diana at Wembley Stadium and the Wimbledon tennis championships.
Police have urged anyone with information to phone the confidential Anti-Terrorist hotline number on 0800789321.