Anti-Heathrow expansion protesters have climbed on to the roof of the Houses of Parliament next to Big Ben.
The group of at least five people unfurled banners, including one saying "No 3rd runway at Heathrow", and threw paper aeroplanes.
The incident comes two days after a similar protest when banners were draped over a plane at Heathrow.
The protesters said they had gained access to Parliament's roof, via a fire escape, using visitor passes.
One of them, who said he was called Richard George, told the BBC they had spoken to police and said they intended to continue the protest until at least prime minister's questions at noon.
The protest will raise fresh questions over security at the Houses of Parliament.
The buildings have been subject to increasingly tight security measures in recent years because of the threat of terrorist attack.
Security had also been reviewed and stepped up after previous protests, including a fathers' rights group throwing purple powder at then prime minister Tony Blair, and anti-hunting ban protesters entering the Commons.
This should be a fun PMQ's, then.
