Climate change activists claim to have got onto the north runway at Heathrow Airport to launch a protest.
A spokesman for direct action group Plane Stupid said a dozen demonstrators cut through a fence and entered the runway at 3.30am.
He said the protesters were campaigning against airport expansion and would stay on the runway for as long as possible.
A video posted online appeared to show several activists chained together and being spoken to by police officers.
Earlier this month a long-awaited report recommended that a new runway should be built at Heathrow rather than Gatwick.
After three years of investigation, the Airports Commission said Heathrow was best placed to provide "urgently required" capacity, but environmentalists warned that building a new runway there will make it harder to reduce air pollution and climate change emissions.