Home Secretary Jacqui Smith said police had searched at least 19 locations as part of a "fast-moving investigation" into what British officials have called an al-Qaida-linked network. Media reports have said two of the seven suspects arrested so far may be doctors working in Britain, including an Iraqi.
In a statement to the House of Commons, Smith urged Britons to remain united.
"Let us be clear: Terrorists are criminals, whose victims come from all walks of life, communities and religious backgrounds," she said. "Terrorists attack the values that are shared by all law-abiding citizens."
Daniel Gardiner, a rental agent whose company leased a Glasgow-area home searched by police, said authorities contacted his firm 10 minutes before the airport attack, saying they had tracked phone records there linked to the two foiled car bombings in central London.
"A card was put through one of my colleague's door, asking if we would contact them," he said.
"A couple of hours later, they (police) came back to us with a name, and we were able to trace their records," he said. "The police wanted to know why we had dialed a certain phone number. They had the phone records from the situation down in London."
Vigilance was already heightened ahead of the anniversary of Britain's first suicide attacks, the July 7, 2005, London transit bombings in which four British-bred Muslims killed themselves and 52 commuters on three subway trains and a bus.
In the latest attacks, two car bombs failed to explode in central London on Friday and two men rammed a Jeep Cherokee into the entrance of Glasgow International Airport on Saturday.
Police in Glasgow said Monday that two more men were arrested the day before in the airport bomb attack investigation. Strathclyde police said two, aged 25 and 28, were detained under the Prevention of Terrorism Act.
Police have declined to identify any of the suspects, but British television and newspapers widely identified one as Mohammed Asha, a doctor working at the North Staffordshire Hospital, near the Midlands town of Newcastle-under-Lyme, where the police searched a house on Sunday. The hospital refused comment.
The man was arrested along with a 27-year-old woman when the police pulled over a car on the M6 highway in northwest England late on Saturday. British media have described a chase that involved 18 undercover police cars along one of the country's busiest roadways.
In Jordan, Asha's brother Ahmed told The Associated Press he had heard the media reports and said his 26-year-old sibling "is not a Muslim extremist, and he's not a fanatic."
"It's nonsense because he has no terror connections," he said.
Britain's Sky News and the British Broadcasting Corp., citing unidentified sources, gave the name of a second doctor arrested as Bilal Abdulla. Police would not comment on the report and the Iraqi Embassy declined comment. According to the British General Medical Council's register, a man named Bilal Talal Abdul Samad Abdulla was registered in 2004 and was trained in Baghdad.
The driver of the Jeep, whose body was in flames after the attack, is under police guard at Paisley's Royal Alexandra Hospital in Glasgow. A 27-year-old man also was arrested at the airport attack by police and was being held at a high-security police station in Glasgow.
Gardiner said his tenant _ the only person listed as living in the house _ was thought to be a doctor at the Royal Alexandra Hospital. A controlled explosion was carried out Sunday on a car left at the hospital that police said was linked to the airport attack.
Police interviewed staff at the Let-It office and took away all documentation about the tenant, he said. They spent Sunday searching the property.
A British government security official said a loose U.K.-wide network appeared to be behind the London and Glasgow attacks but investigators were struggling to pin down suspects' identities.
"These are not the type of people who always carry identity documents, or who use their real identities," the official said on condition of anonymity because of the sensitivity of the inquiries.
In London on Monday, long lines of cars forming behind police checkpoints on the London Bridge. Concrete car-blockers were in place protecting the Wimbledon tennis tournament.
Associated Press reporter Shafika Mattar in Amman, Jordan, contributed to this report.