A group of climate change activists cut through a perimeter fence at London Heathrow airport on Monday to stage a protest on one of its two runways, causing cancellations and delays to flights, officials said.
Thirteen activists from the group Plane Stupid took part, attaching themselves to each other and to fencing with bicycle locks and what look like plaster casts on their arms, photos on their Twitter account showed.
"Nine have been arrested and the remaining four protesters have been removed to a safe area," a police spokesman said, indicating that officers were still trying to unlock them.
The police said they were alerted to the presence of protesters on the runway at 3:45 am (0245 GMT).
The airport said in a statement that both runways were open, although it advised passengers to check their flight status before travelling to the airport.
A video uploaded on the Internet showed several activists attached to a police officer who was telling them: "You are causing severe disruption and it will be in the millions of pounds because it will take us a while to remove you."
British Airways said it had cancelled seven domestic and short-haul flights.
A government-appointed commission earlier this month recommended building a third runway at Heathrow to increase capacity but the issue divides Conservative Prime Minister David Cameron's government and has been criticised by many local residents.
"Building more runways goes against everything we're being told by scientists and experts on climate change," said Ella Gilbert, one of the protesters.
London's five airports form the busiest air hub in the world with around 135 million passengers a year.
Heathrow handled 73.4 million passengers in 2014, making it Europe's biggest airport by passenger traffic.