Protesters are blockading offices belonging to Heathrow operator BAA.
After some campaigners clashed with riot police yesterday as they set up camp at the company headquarters, officers said protesters overnight remained peaceful. Large numbers of police officers stood by in riot vans
Protestors from the climate change camp who are demonstrating outside the offices of BAA at Heathrow.
The protesters erected tents in the car park, but BAA said they were having little impact on the business and were not disrupting operations.
A police spokesman said he was "not aware" of pledges by some protesters to attempt to prevent BAA employees from entering the building.
Last night police prevented some aggressive groups from entering the protest, and three campaigners were arrested.
The campaigners had gathered at nearby Sipson village waving banners bearing slogans such as "You Fly, They Die".
Last night officers at the scene estimated crowd numbers at nearly 1,000, but the number remaining this morning was between 100 and 200.
Also last night, eight campaigners who blockaded a British Airways cargo centre were arrested when they stopped vehicle access to the depot by lying on the ground.
The campaigners say their protest is aimed at highlighting the growing level of greenhouse gases emitted by transporting food by air. Some activists are also opposing the construction of a third runway at Heathrow.