A protest on the roof of Parliament against the expansion of Heathrow has ended after almost three hours.
Campaigners who unfurled banners from Parliament were led away while Gordon Brown was taking part in prime minister's questions in the Commons.
It is thought the group gained access using visitors' passes, and made their way to the roof via a fire escape.
Mr Brown told MPs decisions had to be made "in the chamber of this House and not on the roof of this House".
Protesters from the Plane Stupid group climbed onto the roof at about 0930 GMT, unfurled banners and threw down paper planes, which they said were made from documents obtained under the Freedom of Information Act.
The five protesters were led off peacefully and then searched by officers at about 1220 GMT. They were arrested for trespass and are being questioned by police.
One of them, Richard George, said: "We decided to let Gordon Brown get on with prime minister's questions, but we just wanted him to know what it is like to have an inconvenience above your head that you did not ask for."
The protest coincided with the final day of the government's consultation on the expansion plans - protesters claimed the result had been "fixed".
A Plane Stupid spokesman added: "Two million Londoners face increased levels of noise, while CO2 emissions from the airport would shoot up despite claims by Brown that he's committed to fighting climate change."
It followed a similar episode on Monday when banners were draped over a plane at Heathrow.
Matthew Knowles, spokesman for the Society of British Aerospace Companies, said: "These stunts are becoming tiresome and do nothing more than peddle inaccurate propaganda."
The incident has raised fresh questions about security at Parliament with calls for an immediate inquiry.
It is seen as a fundamental part of British democracy that people should be able to go Parliament to lobby their MP.
The public are allowed temporary passes to some parts of the building, but to get to most parts of the Palace of Westminster it is necessary to have security clearance or to be a guest of someone with clearance.
The Conservative MP Michael Jack told the BBC he felt the protesters may have had some inside help
He said: "It's not the easiest of places to find your way around and for getting on to a roof - I can only think of one particular route so it may be a bit of an inside track on this one."
The buildings have been subject to increasingly tight security measures in recent years because of the threat of terrorist attack.
Security had also been increased after previous protests, including a fathers' rights group throwing purple powder at then prime minister Tony Blair, and people protesting against the hunting ban entering the Commons.
The demonstration lasted nearly three hours
Former Joint Intelligence Committee chairman Sir Richard Mottram told the BBC later he had been surprised that the protesters had got onto the roof.
"The security at the House of Commons has been tightened up and one would not expect to see this happen.
"But of course you have got to have a Parliament that is open to people - I've just walked into the studio going past Parliament and there were lots of legitimate protesters queuing up to get in. It all depends on the basis by which these people got into the building."
Liberal Democrat transport spokesman Norman Baker said it was "clearly worrying" they were able to get onto the roof.
But he added: "Having said that I can understand, in a sense, the objective of the protesters. We have had a sham consultation on Heathrow."
And Labour MP John McDonnell, whose constituency includes Heathrow, added: "If politicians refuse to listen, direct action becomes inevitable."
However a Department for Transport spokeswoman said: "The department has gone to great lengths to encourage as many people as possible to participate in this consultation."
She said public exhibitions had been held, summary documents posted to more than 200,000 homes and more copies had been available from a dedicated phone number.