Changes may be made to Heathrow Airport's security plan, London's Metropolitan Police says.

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Climate change activists chain themselves to gates to protest possible third runway

Police arrest 13 people after flights delayed at London's Heathrow Airport

CNN —  

Climate change protesters, demonstrating against a possible third runway, caused delays Monday at London’s Heathrow Airport.

“A group of people is currently staging a protest on the northern runway,” Heathrow spokeswoman Rachel Betts said during the demonstration. “We are working closely with the police who are dealing with the incident.”

A group calling itself Plane Stupid organized the protest.

Activists chained themselves to gates and displayed banners on the tarmac. One banner said, “No ifs. No buts. No third runway.”

In other images posted by protesters, authorities appeared to use bolt cutters to remove the activists from the gates.

“Both runways are open, although there will still be delays and a few cancellations – we are sorry for the disruption to passengers,” Betts said. “Our priority remains to ensure the safe running of the airport.”

Thirteen protesters were arrested on suspicion of Aviation Act offenses, according to London’s Metropolitan Police.

The incident will be reviewed, and necessary changes will be made to the airport’s security plan, a police statement said.

CNN’s Radina Gigova and Tina Burnside contributed to this report