Two doctors working at UK hospitals are among seven people being held by police investigating the failed car bombings at Glasgow airport and in London.
The others include two men held in the Paisley area near Glasgow on Sunday - the latest to be arrested. Neither of these is thought to be of UK origin.
This evening Stansted Airport in Essex has closed because of a security alert.
It has also emerged that police may have been trying to trace the airport bomb suspects before Saturday's attack.
Daniel Gardiner, director of a Paisley-based letting agency believed to have rented out a house to one of the suspects, said officers had traced his company after tracking phone records linked to the foiled London car bomb attacks.
"The police wanted to know why we had dialled a certain phone number. They had the phone records from the situation down in London," he said.
Police said the latest arrests were of two men aged 28 and 25 and came as a result of "intensive police operations in the Paisley area".
Assistant Chief Constable John Malcolm, of Strathclyde Police, said: "This continues to be a fast-moving investigation and I am grateful to the public for their perseverance and support during these difficult times."
Detectives are linking the failed bombings, and the UK's terror alert system has been raised to its highest level - critical.
In a Commons statement, Home Secretary Jacqui Smith said the UK would not be "intimidated" by "those who wish to destroy our way of life and our freedoms".
She said 19 locations have been searched by police investigating the attacks.
Two of those being held by police are doctors.
Dr Mohammed Asha, 26, who was brought up in Jordan and 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 along with a 27-year-old woman thought to be his wife.
And an Iraqi doctor, Bilal Abdullah, suspected of the attack on Glasgow Airport, worked in Paisley.
It is believed he was employed as a locum at the Royal Alexandra Hospital, just two miles from the airport building attacked on Saturday.
The other man detained at Glasgow airport had severe burns and remains in a critical condition, and under armed police guard, at the Royal Alexandra.
The fifth person, a 26-year-old man, was arrested in Liverpool on Sunday.
Warrants have been issued which allow police to further detain the three people arrested in England - who are being held at Paddington Green police station - until Saturday.
All the first five people held are believed to be of Middle Eastern nationalities.
Dr Asha was described by family and university colleagues in Jordan as a "brilliant" physician who had not been involved with extremist organisations.
Holding a picture of his son, his father Jamil, said he only learnt of the arrest through the media and called on Jordan's King Abdullah II to intervene.
He said his son obtained a medical degree in Jordan in 2004 and came to the UK in the same year to specialise in neurology.
Dr Abdullah is said to have qualified in Baghdad in 2004 and first registered as a doctor in the UK in 2006.
The alert at Stansted started at 1910 BST after a suspect package was found near the front of the terminal.
No one is being allowed in or out and people have been sent to the rear of the terminal. Planes are still taking off and landing.
Detectives are trying to trace the movements of a green Jeep Cherokee - registration L808 RDT - loaded with gas cylinders, which crashed into the doors of Glasgow airport's main terminal and burst into flames on Saturday afternoon.
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.
In other developments:
A controlled explosion has been carried out the Royal Alexandra Hospital, which is being searched as part of the investigation
Commuters and air travellers were being warned to expect extra delays to journeys, with extra police patrols and vehicle searches taking place at airports and transport hubs
The prime minister has held meetings with Scottish Secretary Des Browne, Transport Secretary Ruth Kelly and MPs affected by the attack on Glasgow airport
Gordon Brown's official spokesman dismissed suggestions the attacks were linked to either Mr Brown taking over as prime minister or his Scottish origins, saying: "These attacks could have happened any time, anywhere in the country."
Alex Salmond, Scotland's First Minister, said life was already getting back to normal in Scotland
The United States has increased security at airports, with extra US air marshals on flights to Britain
Houses have been searched in Houston, near Paisley; in Merseyside, and Bradwell and Chesterton, Newcastle-under-Lyme, north Staffordshire
Earlier, the home secretary 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 the 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 has declined to comment on reports that US Intelligence had recently warned the UK that Glasgow could be a target.
Police have urged anyone with information to phone the confidential Anti-Terrorism hotline number on 0800789321.