Heathrow was selected as the site for a new runway in south east England earlier this month, causing disappointment for some local residents and climate change groups who oppose airport expansion.
“Both runways are open although there will still be delays – we are sorry for the disruption to passengers. Our priority remains to ensure the safe running of the airport,” the airport said in an emailed statement.
Plane Stupid, a group which campaigns against airport expansion, said 12 activists had got onto the runway at 2.30am. Pictures posted by activists on Twitter showed them chained together and later being led away by police.
The activists argue that expanding runways such as Heathrow will increase carbon emissions.
“Building more runways goes against everything we’re being told by scientists and experts on climate change,” activist Ella Gilbert, who was on the runway, said in a statement.
Prime Minister David Cameron has said Britain will make a final decision on the £23bn plan for a third runway at Heathrow airport by the end of the year, setting the stage for a battle with environmentalists over the issue.
Previous plans to build a new runway at Heathrow were scrapped in 2010, in part due to environmental concerns.