Greenpeace activists have breached security at Heathrow Airport and climbed on top of an aeroplane.
Four people were arrested after unfurling a banner from the top of the British Airways Airbus A320, which had arrived from Manchester.
BAA said operations at the airport were not affected and described the protest as "unlawful and irresponsible".
The move came as protesters were gathering in Westminster on Monday to oppose plans to expand the airport.
A government consultation on the plan closes on Wednesday.
"We are here to draw a line in the sand and tell Gordon Brown his new runway must not and will not be built"
Greenpeace said protesters put a banner reading "Climate Emergency - No Third Runway" over the plane's tailfin at about 0945 GMT.
It said two women and two men crossed the tarmac at the airport after the passengers had disembarked.
One protester, Anna Jones, said: "Our planet and the people who live on it are in danger.
"Climate change can be beaten but not by almost doubling the size of the airport.
"We are here to draw a line in the sand and tell Gordon Brown his new runway must not and will not be built."
'Four arrests'
BAA said police were attending the incident but it had not disrupted airport operations.
It said: "The government is currently consulting on the future of Heathrow airport and all parties have the opportunity, through the proper democratic process, to make their views known."
But it criticised the Greenpeace protest as "unlawful and irresponsible" and said there would be a full investigation.
The four were arrested and taken to Heathrow police station at about 1100 GMT, a BAA spokeswoman said.
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