British police were hunting on Monday for accomplices of suspected militants who rammed a burning jeep into a Scottish airport and tried to detonate two car bombs in central London.
A police source said a manhunt was underway for an unspecified number of suspects after five people were arrested at the weekend. All five detained were thought to be foreigners, the source said.
Britain's top-selling Sun newspaper identified one of those detained as an Iranian doctor who worked at North Staffordshire Hospital in central England. A spokeswoman at the hospital declined to comment on the case and police would not identify those detained.
British Prime Minister Gordon Brown, facing a difficult test in his first few days in office, said those behind the botched attacks were associated with Al-Qaeda.
Britain's security has been raised to its highest level, "critical", meaning an attack is believed to be imminent.
"We are dealing with a long-term threat," Brown said on Sunday. He said the attacks could not be justified as opposition to Britain's foreign policy.
"Irrespective of Iraq, irrespective of Afghanistan, we have an international organisation trying to inflict the maximum damage on civilian life in pursuit of a terrorist cause that is totally unacceptable to most people."
US President George W Bush praised Brown for a "very strong response" to the attacks.
"It just goes to show the war against these extremists goes on," Bush told reporters at his family's home in Kennebunkport, Maine.
Brown last week replaced Tony Blair, whose 10 years in office were marked by an aggressive stance on security and a foreign policy which strongly supported the United States in Afghanistan and Iraq.
Peter Clarke, the head of London's anti-terrorism police, said authorities were making swift progress in uncovering the suspected terrorist network behind the threats.
"The investigation into these attacks is extremely fast-moving. It is no exaggeration at all to say that new information is coming to light hour by hour," Clarke said.
The plots raised the specter of attacks on London transport two years ago that killed 52 commuters.
Authorities ramped up security measures at airports and extra police officers patrolled rail stations and increased checks ahead of the Monday morning rush-hour.
"The main thing we are doing is asking passengers to continue to be vigilant and go about their business," said a spokesman for Britain's transport police.
On Saturday, police arrested the passenger and badly-burned driver of a Jeep Cherokee who ploughed their vehicle into the entrance of Glasgow's airport and set it alight in a huge fireball.
The attack came 36 hours after police found two Mercedes car bombs packed with fuel canisters, propane tanks and nails parked near a crowded nightclub in London's teeming theatre district.
Police say the London and Scotland incidents are linked. On Sunday they raided a house in an affluent suburb about 10 minutes from the Glasgow airport, where neighbors said two Asian men had moved in just weeks ago.
The other arrests included a 26-year-old man and a 27-year-old woman seized on a major highway in northern England on Saturday, and a 26-year-old man in Liverpool on Sunday.
(Additional reporting by Luke Baker in London)