The British Airport Authority, which runs Heathrow Airport, has said it will not tolerate disruption to flights from a makeshift camp set up to protest against a planned expansion.
BAA accused the campaigners of being irresponsible for staging a protest at a time of heightened security, while the protestors have accused police of using anti-terrorism legislation to harass them.
Dozens of police officers entered the controversial Camp Against Climate Change camp to inspect the site this morning.
Demonstrators set up the camp two days earlier than planned and will take part in what they are terming direct action this weekend.
August is the busiest month of the year at Heathrow. BAA estimates that around 1.5m passengers are due to pass through the airport this week.
Organisers of the protest are expecting up to 2,000 people from across the UK and other parts of Europe to join the week-long event.
The protestors oppose the expansion of the airport that they believe would increase global warming if given the go-ahead.
They say they do not intend to disrupt air traffic, but want to highlight it as a contributor to global warming.
A spokeswoman for British Prime Minister Gordon Brown said peaceful protest was a legitimate right but that any action to disrupt Heathrow would be unacceptable.