Police are now holding four people following an attack on Glasgow Airport that is being linked to the discovery of two car bombs in London's West End.
The UK has moved to its highest level of terror alert - critical - after a burning car crashed into the airport.
Two men were arrested, one of whom is in hospital with severe burns, after the attack on Saturday afternoon.
Two further arrests were made on the M6 in Cheshire overnight, but no further details are available.
An eyewitness to the arrests, Peter Whitehead told BBC News 24 that three cars straddled the motorway and brought traffic to a halt.
"It turned out they were unmarked police cars. In front of them were a couple of other unmarked police cars and they forced a car onto the hard shoulder and got the occupants out and as far as I can see arrested them," he said.
Counter-terrorism police made the arrests hours after Strathclyde police had confirmed they were linking the attack in Glasgow with 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."
Two first two men were arrested at Glasgow Airport, which is in Paisley, after their car crashed into the main doors and burst into flames.
One of the suspects suffered severe burns and was taken to the city's Royal Alexandra Hospital, where he is said to be in a critical condition.
Strathclyde Police had believed he was in possession of "a suspect device", but it was later found not to be explosive.
The vehicle involved has been made safe and will be examined by forensic experts. It is expected to be removed later on Sunday morning.
Two Mercedes containing petrol, gas cylinders and nails were found outside the Tiger Tiger club in London's Haymarket and a nearby street on Friday, but the devices did not detonate.
||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 the light of the attacks, the national terrorism threat level was raised to "critical" on Saturday after a meeting of the government emergencies committee, Cobra.
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.
BBC home affairs correspondent Danny Shaw said the decision to raise the level to critical reflected concern that those responsible have the capability and intent to carry out further bombings.
He said the similarities between the attacks suggested they were carried out by the same individuals or by members of the same cell.
All flights to and from Glasgow Airport were suspended after the attack on Saturday.
And a number of other airports stepped up security, including Edinburgh, Newcastle, Birmingham, Manchester and Blackpool. John Lennon Airport in Liverpool was closed while a suspicious vehicle was removed for examination, but has now re-opened.
Security has also been tightened at airports across the US.
In a televised statement on Saturday, Prime Minister Gordon Brown urged the public to remain vigilant.
"I know the British people will stand together, united and resolute," he said.
He also thanked the police, the security and emergency services for their "dedicated professionalism" in Glasgow and London.
Police in London are checking CCTV footage in their investigation into the planting of the two car bombs.
Police have been increasing patrols and security for events in London over the weekend, including the Gay Pride parade on Saturday, the Concert for Diana at Wembley Stadium on Sunday and the ongoing Wimbledon tennis championships.
Police have urged anyone with information to phone the confidential Anti-Terrorist hotline number on 0800789321.