Campaigners angry at plans to expand London's Heathrow Airport have climbed on to the roof of the Houses of Parliament in the centre of the city.
The protesters let banners down the side of the building, with one saying "No 3rd runway at Heathrow".
They think the increase in flights that will come with the new runway will be really bad for the environment.
The protesters, thought to be from a group called Plane Stupid, later came down and were being quizzed by police.
Their protest came just two days after campaigners from environmental group Greenpeace draped banners over a plane at Heathrow.
Two men and two women clambered on top of the aircraft after it landed on a flight from Manchester on Monday.
The campaigners are angry because planes use a lot of fuel, which harms the environment.
The rooftop protesters said they managed to get on Parliament's roof by a fire escape, using visitor passes to get into the building.
Over recent years, security has been tight at the Houses of Parliament - the UK's government headquarters - because of the threat of terrorist attacks.
The protest will raise fresh concerns over security at the building.