Heathrow Airport cancelled some flights on Monday after at least 12 activists cut through a perimeter fence and chained themselves together on a runway to protest against the possible expansion of Britain's biggest airport.
In a stunt which raised questions about the security of Europe's busiest airport, the activists said it took only minutes to cut through an apparently unguarded wire fence at around 0230 GMT (03:30 a.m. BST) granting them access to the northern runway.
"Both runways are open, although there will still be delays and a few cancellations – we are sorry for the disruption to passengers," Heathrow said in an emailed statement.
The activists, from a group called 'Plane Stupid' which opposes the growth of aviation, said a 23 billion pound ($36 billion) plan to build a third runway at Heathrow would increase carbon emissions and be blocked by further protests.
Pictures posted by activists on Twitter showed them chained together, some smiling under blankets at dawn and lying beside the black scars left by the tires of landing jets.
Later pictures showed some being led away by British police, who said that six had been arrested and about seven activists remained on the runway some six hours later.
"Officers remain on scene, continuing to work to remove the remaining protesters," said a spokesman for London's Metropolitan Police. The protesters are far enough away to allow most planes to take off.
At a time when Britain is on its second highest alert level of "severe" -- meaning a militant attack is considered highly likely -- the incursion on to the runway also underscored the challenge in guarding a 1,227 hectare (3150 acre) airport with miles of fences and a maze of hangars.
The disruption indicates the potential challenge the airport will face if Prime Minister David Cameron decides to grant Heathrow a third runway, a decision he is due to take by year end.
Heathrow was selected as the site for a new runway in south east England earlier this month and ministers say Britain needs at least one new runway if London is to remain the dominant city in its time zone.
For decades, politicians have discussed where to build the runway but have shied away from a final decision that they expect will provoke a vociferous campaign by environmentalists and residents.
The runway "was accessed through the fence. It took a matter of minutes," Joshua Virasami, an activist who said he was involved in the planning of the protest, told Reuters.
"No if, no buts, airport expansion will not happen, and we will ensure that," he said. Another activist told Reuters that no guards were around when they cut through the fence.
Previous plans to build a new runway at Heathrow were scrapped in 2010, in part due to environmental concerns.
Heathrow's largest shareholder is Spanish infrastructure firm Ferrovial. Other partners include Qatar Holding, China Investment Corp and the Government of Singapore Investment Corp.