A day of direct action by climate change protesters has failed to disrupt Heathrow Airport.

A number of the campaigners, who object to plans to build a third runway, clashed with riot squads today as they tried to lay siege to the headquarters building of BAA, the airport operator.

The demonstrations near Heathrow Airport had been largely peaceful so far.

The environmentalists are highlighting what they say are the links between aviation and climate change.

For nearly a week climate change campaigners have been camped in a field just north of the world's busiest international airport.

Dozens of protesters have been arrested since the camp was set up, mainly for minor offences. The campaigners have taken their protests to private airports and government departments in London but are saving their main protests for today at Heathrow.

Up to 2,000 environment activists are expected to join the day of direct action but the campaigners will not say what they are planning.

London's Metropolitan police believe a hardcore minority will use the event to confront the police and try to disrupt the airport on one of its busiest days.