Many planes have been grounded at Heathrow BAA advice

Armed police are patrolling terminals and travellers are undergoing searches.

Heathrow airport has been the worst affected, although the ban on short-haul flights into the airport was lifted on Thursday afternoon.

Speaking at Heathrow, Tony Douglas, from airport operator BAA, said it would be "open as usual" on Friday.

"It's been open all day today and it will continue to be open tomorrow and going forward," he told BBC News on Thursday night.

New security restrictions would remain in place, he added.

"The message for passengers is very clear.

"Don't bring carry on luggage and don't bring personal belongings unless they fit the list that's been provided and presented in a clear plastic bag.

"And please, most importantly, check with your airline that their schedule's being operated tomorrow or over the weekend before you come to the airport.

"Finally, allow enough time for the new security processes."

All cabin baggage must be processed as hold baggage and carried in the hold of passenger aircraft departing UK airports Advice to passengers At a glance: disruption New luggage guidelines Send us your experiences

A Department for Transport spokeswoman said the government hoped the current hand baggage restriction would be in place only for a "limited time".

The measures were "proportionate to the threat now" and would continue "only as long as the situation demands", she added.

Earlier, sources close to Transport Secretary Douglas Alexander indicated restrictions could become permanent.

One source told the BBC the "way we travel will never be the same again".

Mr Alexander will chair a meeting of the National Aviation Security Committee on Friday afternoon to consider security measures at airports and on planes.

Thousands disappointed

Despite Thursday's disruption, many passengers have praised airlines for keeping them informed and said they understood why the measures had been introduced.

Security has been increased at Channel ports and the Eurotunnel terminal.

Eurostar said it had taken at least 5,500 extra passengers to European destinations from London Waterloo on Thursday as a result of "displaced airline passengers".

This was 24% up on normal business, a spokesman said.

British Airways cancelled more than 360 flights at Heathrow by mid-afternoon, while easyJet and Ryanair also had to disappoint thousands of travellers by cancelling flights.

Many holidays have been ruined by the delays

The BBC News website's Krishan Rama at Heathrow said travellers were not as worried by the security operation as by the inconvenience of being allowed to take just passports and medicines on the plane.

Courteney Dane, who is travelling back to Australia, said: "I'm worried about the cost and the fact I've got a 24 hour flight with nothing to read. I'm not really worried about the security, it's just a nuisance."

Charlotte Demant, a make-up artist, has been in Ghana for two weeks and was on her way home to Denmark.

She said: "I can understand why they are worried about security but the airline has not been helpful.

"We should have at least been given something to eat and drink and money to phone home. I'm going to end up spending the night here in the cold airport."

By late afternoon, queues continued to build around terminal four as passengers packed into the building.

Armed police have been drafted in to many airports. Sussex police said extra officers were on duty, adding: "We appreciate that there is major disruption at and around Gatwick airport today but please be assured that we are responding to a very serious risk and that the action being taken is absolutely necessary to minimise that risk."

Increased security

Scotland's airports were busy throughout the day but passengers are still being advised to turn up even though there may be long queues for check-in.

All hand baggage will now have to be checked in with only a small number of essential items allowed through search controls Douglas Alexander West Midlands disruption Hold-ups at Stansted Disruption at Gatwick

Commentator Murray Walker was one of those stuck at Aberdeen. "I was supposed to be flying to Heathrow and am hoping to transfer to Birmingham, I have got to get to Silverstone.

"It's better safe than sorry, but I have a problem with those who are making this happen."

Felix Eza, from Failsworth in Manchester, was philosophical about the delay to his journey from Manchester to Orlando, Florida.

"If they [the terrorists] achieve their objective to disrupt our lives then it's a problem because they have won," he said.

Passengers are being asked to be patient when facing delays

Announcing the security measures on Thursday morning, Mr Alexander outlined the steps passengers needed to take.

"What these changes mean in practice is that all hand baggage will now have to be checked in with only a small number of essential items allowed through search controls," he said.

"Exceptions will be in place for those travelling with infants and for prescription medicines."

Passengers are being asked to check in most items of baggage, apart from wallets and purses, identity and travel documents, prescription medicines, spectacles and sunglasses and keys.

Those travelling with children will be allowed items essential for caring for them on the flight.