Britain was on Sunday on its highest level of terror alert following an attack on the Glasgow airport and foiled car bombings as four persons were arrested for the incidents and police searched for more suspects in the Scottish city of Renfrewshire.
Prime Minister Gordon Brown said Britain 'will not yield despite a sustained threat from people associated with Al Qaeda' as police said the attack on the airport was believed to be linked to the three car bombs found in London.
The country moved to its highest level of terror alert -- critical -- after a burning car crashed into the airport and the government's emergency response unit, Cobra, was to meet to review the situation.
The critical threat level indicates terror attacks are 'imminent'.
Four persons have been arrested in connection with the attack at Glasgow. Two men were arrested at the airport after their Jeep Cherokee crashed into the main doors and burst into flames.
Two more arrests were made on the M6 motorway in Cheshire on Saturday night.
One of the suspects in the airport attack suffered severe burns and was in a critical condition in hospital.
Police searched several houses in Renfrewshire in connection with the attack on the airport.
'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', a police statement said.
Brown told BBC, it was 'clear that we are dealing, in general terms, with people who are associated with Al Qaeda'.
"It's obvious that we have a group of people-- not just in this country, but round the world-- who're prepared at any time to inflict what they want to be maximum damage on civilians, irrespective of who the religion of these people who are killed or maimed are to be," Brown said.
Counter-terrorism police made the two arrests from the M6 motorway hours after Strathclyde police said they believed the attack in Glasgow was linked to the events in London.
Chief Constable Willie Rae said, "There are clearly similarities, and we can confirm that (the Glasgow attack) is being treated as a terrorist incident."
All flights to and from Glasgow airport were suspended after the attack.
The airport was being reopened in phases and flights from Orlando and Ibiza had arrived.
A number of other airports stepped up security, including Edinburgh, Newcastle, Birmingham, Manchester and Blackpool.
Security has also been tightened at airports across the US. Police in London are checking CCTV footage in their investigation into the planting of the two car bombs.