Police are urgently hunting at least one person over attempted car bombings in London and at Glasgow airport.
Five people - none thought to be British - have been arrested so far in Paisley, Liverpool and Cheshire.
Houses are being searched in Houston, near Paisley, Merseyside, and in Staffordshire, where one of two doctors arrested in the inquiry lived.
Police are linking the failed bombings and the UK remains on high alert amid fears of a possible further attack.
On Monday, Staffordshire Police closed off Priam Close, in Bradwell, not far from Chesterton, where one of the people being held, Dr Mohammed Asha, lived.
Dr Asha, who worked as a junior doctor at the Royal Shrewsbury Hospital and the Princess Royal Hospital in Telford, was arrested on the M6 in Cheshire on Saturday night.
The terror alert level was raised to critical - its highest level - after a Jeep Cherokee, loaded with gas cylinders, crashed into the doors of Glasgow airport's main terminal and burst into flames on Saturday afternoon.
Home Secretary Jacqui Smith defended the decision not to raise the terror alert level to critical earlier in the light of suggestions that the new Brown administration could be a target.
She told the BBC that decisions on the level of alert were taken by an independent committee, which properly raised the level to critical after attempted bombings.
She said the government had put sufficient focus on counter-terrorism and was looking at ways of "tackling the ideology that recruits people to terrorism" and at further legislation.
Ms Smith, who will make a statement to MPs in the Commons on Monday afternoon, also said she was encouraged by the progress of the police investigation into the attacks.
"There's clearly progress being made," she added.
Earlier, Ms Smith had declined to comment on reports that US Intelligence had recently warned the UK that Glasgow could be a target.
Alex Salmond, Scotland's First Minister, told BBC Radio 4's Today programme that life was already getting back to normal in Scotland.
Paisley - Burning car driven into doors of Glasgow Airport on Saturday. Two people arrested at scene
London - Two car bombs that failed to detonate found in West End on Friday
Paisley - Controlled explosion carried out on car at Royal Alexandra Hospital
Cheshire - Man and woman arrested on M6 on Saturday night, then taken to London for questioning
Liverpool - Man arrested. Police searching two addresses
Houston - Police searching houses in the Renfrewshire village, near Glasgow Airport
Newcastle-under-Lyme - Police searching house in Staffordshire
"The prime minister and I have been at one in making sure that the country will not be intimidated or stopped from going about its business by terrorist incidents," he said.
It has emerged that detectives tried to contact a letting agency, believed to have rented out a house to one of the suspects, two hours before the attack at Glasgow airport.
Daniel Gardiner, director of the Paisley-based Let-It agency, said officers had traced his company after tracking phone records obtained in London.
The attempted attack in Paisley came a day after two Mercedes containing petrol, gas cylinders and nails were found outside a nightclub in London's Haymarket and in a nearby street.
The devices failed to detonate.
Commuters and air travellers were being warned to expect extra delays to journeys on Monday, with extra police patrols and vehicle searches taking place at airports and transport hubs.
Detectives are trying to trace the movements of the green Jeep Cherokee - registration L808 RDT - which crashed into the airport.
BBC home affairs correspondent Danny Shaw suggested the need to track the vehicle's movements could mean the police were still not sure where the car bomb's components were picked up and assembled and who else was involved.
A 26-year-old man, identified by the BBC as Dr Asha, and 27-year-old woman were arrested on the M6 near Sandbach, Cheshire, on Saturday night.
Dr Asha and the woman have been taken to Paddington Green police station in London for questioning.
Their arrests are linked to the search of a house at Newcastle-under-Lyme in Staffordshire.
Another 26-year-old man was arrested in Liverpool on Sunday and remains in custody at a Merseyside police station.
A man arrested by police at Glasgow airport suffered severe burns
Two men were detained at Glasgow Airport on Saturday after the attempted attack.
One had severe burns and remains in a critical condition, and under armed police guard, at Royal Alexandra Hospital, in Paisley.
The BBC has learned those arrested are believed to be of Middle Eastern nationalities.
On Sunday, police also searched addresses in the Liverpool area and in Houston, Renfrewshire, close to Glasgow Airport.
The UK's top counter-terrorism officer, Deputy Assistant Commissioner Peter Clarke, of the Metropolitan Police, said the links between the three attempted car bombings were becoming "ever clearer".
He described the investigation into the failed bombings as "extremely fast-moving", with forensic searches of vehicles proving "extremely valuable" and thousands of hours of CCTV being sifted through.
In other developments:
Prime Minister Gordon Brown said it was "clear that we are dealing, in general terms, with people who are associated with al-Qaeda"
A controlled explosion was carried out on a car at the hospital where a suspect is being treated. It is thought to have been connected to the failed airport bombing, police said
United Nations secretary general Ban Ki-moon said in a statement that he "deplored" the attempted attacks and said he would discuss them with Mr Brown when he visited London next week
US President George Bush said the failed bombings showed "the war against these extremists goes on". He praised "the very strong response" of the UK government
Police have urged anyone with information to phone the confidential Anti-Terrorism hotline number on 0800789321.