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Heathrow protesters illegally blocked a runway in the early hours of Monday (July 13) morning.
Campaigners from anti-airport expansion group Plane Stupid have caused huge disruptions to passengers flying in and out of the airport.
Dozens of flights have been cancelled and many more are experiencing delays after 13 protesters staged a sit-in, chained to the fences on the northern runway.
Ella Gilbert, one of the activists on the runway, told the Mirror: "Building more runways goes against everything we're being told by scientists and experts on climate change.
"This would massively increase carbon emissions exactly when we need to massively reduce them, that's why we're here.
"We want to say sorry to anyone whose day we've ruined, and we're not saying that everybody who wants to fly is a bad person.
"It's those who fly frequently and unnecessarily who are driving the need for expansion, and we cannot keep ignoring the terrifying consequences of flying like there's no tomorrow.
"No ifs, no buts, no third runway. And we mean it."
Police have arrested 13 people on suspicion of Aviation Act offences and taken them into custody.
A Metropolitan Police spokesman said: "At approximately 3.45am on Monday (July 13), officers were alerted to a number of protesters who had unlawfully made their way airside at Heathrow Airport.
"There were 13 protesters in total on the north runway. All 13 have been arrested on suspicion of Aviation Act offences and remain in custody.
"The Metropolitan Police Service works closely with Heathrow Airport Limited to keep those who use or work at Heathrow safe and secure.
"The Metropolitan Police and Heathrow will jointly review the protest incident at the airport and make necessary changes to the security plan."
Activists blocked the runway in response to the recent announcement of the Davies Commission to recommend a third runway at Heathrow.
British Airways has reported delays and cancellations and is urging passengers to check the status of their flight before travelling.
People have taken to Twitter to voice concerns about "expected delays" to flights because of the demonstrators.
Heathrow have apologised for the disruption and said its priority remains the safe running of the airport.
A Heathrow Airport spokesman said: "A group of people have breached the airport perimeter fence and are currently staging a protest on the northern runway.
"We are working closely with the police who are dealing with the incident. The southern runway remains open.
"Our priority remains to ensure the safety of the airport community, including passengers, employees and protesters."