Downing Street has warned climate change protesters near Heathrow that any disruption to the running of the airport would be "unacceptable".
Up to 2,000 people are expected to join the protest
Up to 2,000 people are due at the Camp for Climate Action, with 250 already at the week-long protest against Heathrow expansion plans and aviation generally.
Protesters say "mass direct action" planned for Sunday will be peaceful and will not involve invading the runways.
No 10 said people had a right to protest but must not cause disruption.
More than 20 police officers inspected the site on Monday morning, with about four remaining on the site.
Chief Superintendent Ian Thomas said the protesters were on the site - a sports ground belonging to Imperial College London - illegally.
A spokeswoman for the college said it hoped the protest would be "peaceful, safe and not damage the land".
She said the college had not given permission for the protest and was reserving the right to claim damages or costs from those on the land.
"Any person who gains access to and/or occupies these fields is doing so unlawfully and should vacate the field," she added.
The camp is near the villages of Sipson and Harlington, between the M4 motorway and the airport's northern perimeter.
A number of tents were standing on the site by Monday as protesters from the UK and abroad gathered to highlight their claims that the growth in air travel is a major factor in greenhouse gas emissions.
Organisers say the first few days will be taken up with 100 workshops on issues ranging from campaigning skills to practical training on how to take direct action.
But a website supporting the camp has also promised acts of "civil disobedience".
A fifth terminal is due to open at Heathrow airport in March 2008, and a new runway has been proposed by the government for about 2020.
The airport caters for almost 70 million people each year, and BAA has warned it will not allow passengers to be "harassed or obstructed".
Logistics director, Shaun Cowlam, said he was concerned the protest could distract police from counter-terrorism.
He said: "We recognise their legitimate concerns about climate change.
"But equally we've seen in their past demonstrations that they have interfered with operational issues, and here at Heathrow, particularly with this difficult security time at the moment, we don't believe Heathrow is a particularly valuable place for them to demonstrate."
Gemma Davis, a spokeswoman for the Camp for Climate Change, told the BBC that the intention was not to delay holidaymakers.
"We're not here to try to disrupt passengers, we're here to try to disrupt BAA," she said.
Police have said they expect protesters to use "lock-ons", where people attach themselves to vehicles and fences, drop banners over buildings and engage in sit-down protests across major roads.
About 1,800 officers from Surrey police, Thames Valley police, the Met, and British Transport Police will oversee the event.
Commander Jo Kaye, who is responsible for the operation - codenamed Hargood - said policing the protest would not affect day-to-day counter terrorist operations at Heathrow.
John McDonnell, Labour MP for Hayes and Harlington which includes Heathrow, said the local community "wholeheartedly" supported the protest.
He said: "There is a very strong sense of anger here. This will give us the opportunity to tell people what a third runway will mean to this community.
"The threat is that up to 4,000 homes will be rendered unliveable by air pollution or demolished."
Last week, BAA won a High Court ruling banning certain protest groups from the airport.
Earlier, one of those, Plane Stupid, protested on a barge transporting the wing of an Airbus A380.
Six campaigners pitched tents under the wing as it was about to be taken along the River Dee from the Airbus factory in Broughton, in Flintshire, north Wales.
North Wales Police said the protesters were moved on after about an hour.