Anti-terror police are continuing to quiz 22 people over an alleged plot to blow up transatlantic jets.
Armed police have been out in many UK airports
The Met Police has been granted warrants for the further detention of the suspects until next Wednesday.
A hearing regarding the detention of one individual was adjourned until Monday, and one person has been released with no further action.
The suspects were held following raids in London, High Wycombe in Bucks, and Birmingham, early on Thursday.
The person whose warrant hearing was adjourned until Monday will remain in custody over the weekend.
Those held are suspected of involvement in a plot to blow up airliners travelling from the UK to the US, possibly using liquid explosives hidden in hand luggage.
Airport security has been stepped up in both countries, with stringent luggage searches and restrictions on the carrying of liquids on board.
Although services are beginning to return to normal, travellers are being warned to check with airlines before they set off.
Officials in Pakistan said security forces in the country had also arrested two British men of Pakistani origin in connection with the alleged plot, who were picked up in Lahore and Karachi last week.
The Pakistan Foreign Ministry has identified one of the men as Rashid Rauf.
"There are indications of an Afghanistan-based Al-Qaeda connection," a spokesman said.
A Pakistani security official told Reuters he had been put under surveillance after Britain had tipped off Islamabad that he was in Pakistan.
NEW THREAT LEVELS
Low - an attack is unlikely
Moderate - an attack is possible but not likely
Substantial - strong possibility of an attack
Severe - an attack is highly likely
Critical - an attack is expected imminently
"He has been staying here for quite some time and he has been under strict surveillance since then," the unnamed official added.
Rashid Rauf is thought to be related to Tayib Rauf, 22, who was arrested in Birmingham.
He is among the 19 suspects who have had their assets frozen by the Bank of England.
Britain's security threat level remains at "critical" and investigations are continuing.
Detectives are still searching properties where the arrests took place. Officers have also seized computer equipment from three internet cafes in Berkshire - one in Reading and two others in Slough.
Speaking to chief constables on Saturday, Home Secretary John Reid said no-one should imagine that the recent arrests ended the current terror threat.
"This isn't a time for complacency or self-congratulation. As I have said all along, no-one should be under any illusion that the threat ended with the recent arrests. It didn't. The threat, as well as our efforts, is ongoing," he said.
"The initial targets - the main suspects - have been successfully apprehended, but all of us know that this investigation hasn't ended. I know there is a huge amount to be done and that presents enormous challenges.
"So, we're not yet at the stage where we can or should stop searching. That is why the alert level remains at critical as a precaution."
Meanwhile, British Muslim groups have written an open letter to the prime minister calling for "urgent" changes to the government's foreign policy, because its stance on Iraq and the Middle East is putting civilians at increased risk in the UK and abroad.
Sadiq Khan, one of three MPs to sign the letter along with three peers and 38 groups, said "a sense of injustice" caused by the current approach "plays into the hands of extremists".
Responding to the letter a Downing Street spokesman said: "We should always remember that the terrorism affecting the West today has blighted Muslim countries for several decades.
"It certainly pre-dated our decision to support democracy in Afghanistan and Iraq and of course the September 11 attacks. Our foreign policy is focused on supporting the people of those countries in their desire to live in a democracy just as we enjoy in the UK."