Two Just Stop Oil activists have been arrested after they sprayed orange paint on departure boards at Heathrow Airport.
The Metropolitan Police said the pair were held on suspicion of criminal damage. Officers "remain in the area to deal with any further offences", the force added.
The incident unfolded inside Terminal 5 on Tuesday morning, leaving travellers shocked. A spokesperson for Heathrow said: "Working with partners we have quickly resolved a protest incident in Terminal 5 and all involved have been removed from the airport."
"The airport continues to operate as normal and passengers are travelling as planned. We are in full agreement that the aviation industry needs to decarbonise, but unlawful and irresponsible protest activity is not the way forward and will not be tolerated."
Climate activists have repeatedly targeted airports around the world in recent days, in a campaign named Oil Kills. Just Stop Oil said 21 groups across 12 countries have taken action at 18 airports so far.
Phoebe Plummer, 22, one of the arrested activists, said: "People around the world are rising up to demand an end to oil by 2030. This is an international problem, so ordinary people are doing what our politicians will not, working together globally to put a stop to the harm and suffering that fossil fuels cause.
"Repression and prison time will not stop people stepping up to defend our families and communities. We have to put a stop to oil and gas. If we want to protect life and what we love, we need a treaty to Just Stop Oil by 2030."
Jane Touil, 58, one of the individuals arrested, expressed her frustration, saying: "Ordinary people have to stand up and make their governments do the right thing, because without pressure from us, they won't. I feel so angry and betrayed that politicians have let this happen when they've known about climate breakdown for over 50 years."