Three terrorist suspects were in police custody Sunday - and a fourth man was under guard in hospital - after a flaming Jeep crashed into a Scottish airport and two car bomb plots were foiled in central London.
Scotland Yard said two people had been arrested in Cheshire, in northern England, in a joint swoop by specialist officers from London and Birmingham.
In Scotland, officers arrested two men - one of them badly burned - after a Jeep Cherokee rammed into Glasgow airport and burst into flames.
The green SUV shattered glass doors at the terminal entrance, stopping within meters of where holidaymakers were lined up at check-in counters.
Police and security officials said the attacks were clearly linked, adding all three vehicles had carried large amounts of flammable materials - including gasoline and gas cylinders.
The chaos over the past two days has raised fears that the type of car bomb attacks that have become commonplace in Iraq has now reached European shores. Late Saturday, Britain raised its security alert level to critical - the highest possible level, indicating terror attacks may be imminent.
"This weekend's bomb attacks signal a major escalation in the war being waged on us by Islamic terrorists," said Lord Stevens, London's former police chief who was this week appointed terrorism adviser by Britain's new prime minister, Gordon Brown.
"Now al-Qaida has imported the tactics of Baghdad and Bali to the streets of the U.K.," he said in a column in Sunday's News of the World newspaper.
Police foiled a car bomb plot early Friday in central London, discovering explosives packed into a Mercedes outside a nightclub near Piccadilly Circus and in another car parked nearby.
Glasgow police chief Willie Rae announced the incidents were connected and said a suspect device had been found on a man wrestled to the ground by officers at Glasgow airport and hospitalized in critical condition with severe burns.
"There are clearly similarities and we can confirm that this is being treated as a terrorist incident," Rae said.
British media said the man was wearing a suicide belt and that police had found propane gas cylinders in the vehicle in Glasgow. But police later said an initial inspection by explosives experts had not found a suicide vest. Rae made no mention of gas cylinders.
London police said the latest arrests were in connection with both attacks, but did not elaborate. Officials declined to say whether those arrested were men or women, or whether either suspect was believed to be a person seen running from one of the explosive-laden cars in London.
On Sunday morning, the Glasgow airport was reopened to flights and to travelers who entered the terminal through a police cordon.
John Lennon Airport, in the northwestern city of Liverpool, also was closed overnight after a suspicious vehicle was found nearby. The vehicle was taken away for forensic examination and the airport was reopened at 4:40 a.m. (0340 GMT), police said.
The new terror threat presents Brown with an enormous challenge just three days after taking office, and comes at a time of already heightened vigilance one week before the anniversary of the July 7, 2005, London transit attacks. Those were largely carried out by local Muslims, raising ethnic tensions in Britain.
"I know that the British people will stand together, united, resolute and strong," Brown said Saturday in a televised statement. He defended raising the alert level, which has not been at critical since the August 2006 plot to blow up several trans-Atlantic flights was discovered and made public.
The airport attack left passengers shaken and stranded on the first day of summer vacation for Glasgow schools. All flights from the airport were suspended. At the time of the crash, the airport was bustling with families heading out on holiday.
Meanwhile in London, police prepared to question the two suspects arrested in northern England.
Officers were also reviewing closed-circuit television evidence, as forensics experts searched for clues into the foiled bombings early Friday. The two Mercedes cars had been loaded with gasoline, gas canisters and nails in one of the capital's busiest areas.
Detectives said they were keeping an open mind about who the bombers were, but terrorism experts said the signs pointed to a cell linked to or inspired by al-Qaida.
One former top British security official said terrorists appeared to be trying to take advantage of the inexperience of the new government.
"This is a very young government, and we may yet see further attacks," Dame Pauline Neville-Jones, former head of Britain's joint intelligence committee, told Sky News television.
U.S. President George W. Bush was being kept informed of the situation, the White House said.
A British government security official said methods used in the airport attack and Friday's thwarted plots in London were similar, with all three vehicles carrying large quantities of flammable materials.
The official spoke on condition of anonymity because of the sensitivity of the information.
He said it was too early to be sure whether the suspects had been aided from overseas, but that officials would investigate international links.
Police did not say whether the SUV that struck Glasgow airport was carrying explosives, but one witness reported seeing a gas canister in the vehicle.
Witness Lynsey McBean said one of the men took out a plastic gasoline canister and poured a liquid under the car. "He then set light to it," said Bean. She said the Jeep struck the front door of the airport but got jammed.
"They were obviously trying to get it further inside the airport as the wheels were spinning and smoke was coming from them," she said.
The incident carried reminders of a foiled plot in December 1999 to attack Los Angeles International Airport, when customs agents stopped an Algerian-born man in a car packed with 124 pounds (56 kilograms) of explosives. He was later jailed for 22 years.
The events also hold similarities to recent thwarted plots in Britain. Last year, a 35-year-old British convert to Islam was convicted of plotting to bomb several U.S. financial targets and luxury London hotels, using limousines packed with gas tanks, napalm and nails.
Accused members of an al-Qaida-linked terror cell were convicted in April of plotting to blow up the Ministry of Sound nightclub, one of London's biggest music venues.