Protestors from the environmental group Plane Stupid have staged a demonstration at London Heathrow Airport following the recommendation of a third runway there.
Flights were cancelled this morning after demonstrators cut through a wire fence and got on to the north runway earlier.
Six protestors have been arrested for offences under the Aviation Act, police said.
The runway was closed for three hours, with 13 flights cancelled as a result.
Officials at Heathrow apologised for the disruption and advised passengers to check with their airlines before they travel.
The southern runway remained open throughout the protest.
The Met Police said the protesters had chained themselves together using a tripod which they had assembled themselves.