A security review has been launched after protesters chained themselves to the roof of the Houses of Parliament in London.
The demonstration was suspected to be an inside job, with all the protesters believed to be guests of Parliament pass-holders.
The campaigners from climate action group Plane Stupid were demonstrating against the expansion of Heathrow airport.
They climbed onto the roof of the Palace of Westminster and unfurled two banners, one saying "BAA HQ" and a second which says "No Third Runway".
Scotland Yard said police had been called at 9:30 a.m. local time.
"At this early stage we believe there is a strong possibility the protesters are guests of pass holders," a spokesman said.
Westminster sources believe the banners were stored inside the complex for the protesters to collect. The flags are believed to be too big for the activists to have taken them through security.
Prime Minister Gordon Brown delivered a message to the protest group during a Q & A session.
He said "decisions in this country should be made in the chamber of this House and not on the roof of this House."
Leo Murray, one of the protesters spoke to Sky News from the roof of the Palace of Westminster. He said the group had had a cup of tea in the cafe and then taken the elevator up to the rooftop.