Environmental campaigners have scaled the roof of the Houses of Parliament to protest against the government's apparent support for Heathrow expansion.
Five activists from the campaign group Plane Stupid have breached security at Westminster to unfurl banners protesting against the proposed third runway at Heathrow.
One reads "BAA headquarters" and the second demands "No Third Runway".
Protester Richard George, 27, explained the protest is designed to highlight concerns with the government's consultation on Heathrow, branding Gordon Brown a "weak" prime minister.
The protesters have been making paper airplanes out of confidential Whitehall papers, which they claim show the consultation was "fixed".
Plane Stupid claims ministers have already decided to support the proposed third runway and colluded with BAA throughout the consultation.
Mr George said: "I'm standing on the roof of parliament because the democratic process has been corrupted. The aviation industry has taken full advantage of a weak prime minister to get the Heathrow consultation fixed.
"It doesn't even consider global warming, despite everything Brown has said about the environment and despite the massive impact that aviation has on the climate."
Fellow activist Leo Murray told Sky News the protest was designed to highlight the "absurdly undemocratic" British government.
BAA refused to comment on the protest, branding it a "stunt".
A spokesperson added: "We are very happy to engage in serious debates on airport expansion".
It follows Monday's protest by Greenpeace activists who scaled a plane at Heathrow airport to display anti-expansion banners.
It is understood the protesters entered parliament on guest passes and gained access to the roof through a fire escape.
The Metropolitan police, however, said there was a "strong possibility" the protesters are guests of passholders.
Nevertheless, the protest will spark security concerns, with parliament supposedly protected against terrorist attacks.
Since 2005 it has been illegal to protest within one kilometre of Parliament Square because of the supposed security risk.