Climate change demonstrators were blockading BAA offices near Heathrow Airport as the climax of a day of action after a week-long protest.

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

The 24 hours of direct action began at noon when protestors marched out of their Camp for Climate Action, at nearby Sipson village.

Some trouble-makers were contained by officers near the BAA building, but better-behaved campaigners were allowed into the car park where some began erecting tents. About 350 campaigners gathered outside the building.

A spokesman for the camp said: "The idea is to occupy the BAA site for as long as we can. We will be giving workshops on climate change."

The camp was set up to oppose Heathrow's planned expansion and aviation generally because members, squatting on land owned by Imperial College, believe carbon-emitting activities, such as air travel, cause climate change.

A spokesman for BAA said: "It's currently not impacting on operations at all. If they want to spend the night in the car park, as long as they do not interfere with passengers, we are reasonably relaxed."

A Scotland Yard spokesman said three protestors were arrested - for suspected drugs possession, intent to cause criminal damage and assaulting police. They were taken to West Drayton police station.

The total arrests made, since protests at Heathrow and Biggin Hill airports began on Tuesday, is now 50, police said.

Before the gathering at BAA, protestors marched through Sipson to the site of Heathrow's proposed third runway.

Scores of police linked up to ring-fence one group of about 50 volatile protesters.

Mounted police observed as a larger, more peaceful, group followed.

Banners carried slogans such as "You Fly, They Die" and "No Third Runway: Sipson Village RIP".

Officers at the scene estimated 1,000 people were taking part, although a Scotland Yard spokesman put the official estimate at 400. Police appeared to split the march into the separate groups of protestors as they headed from the runway site towards BAA.

Some campaigners were held in the village of Harmondsworth for some time by police.

One group of protesters was blocked by riot officers as they tried to cut across fields between the camp and BAA, away from routes police wanted them to use.

Skirmishes took place in a residential area near the BAA building as police armed with shields and batons cut off a few protestors who tried to run through.

Other troublemakers were corralled into a circle of riot officers near the BAA building.

Eight campaigners blockaded British Airways' World Cargo Fresh Perishables Centre at Hatton Cross shortly after 9pm last night, police said.

They stopped vehicle access to the Eastern Perimeter Road depot by lying on the ground, locked together in a ring with arm tubes. A camp spokesman said it was aimed at highlighting the growing level of greenhouse gases emitted by transporting food by air.