A climate change protest camp is being set up outside Heathrow airport, two days earlier than had been expected.
Protesters will attend a week of activities
Thousands of protesters are expected this week at the Camp for Climate Action, which opposes the expansion of London's biggest airport.
About 150 campaigners have begun building a camp less than a kilometre outside the airport's perimeter.
In response, airport operator BAA warned it would not allow passengers to be "harassed or obstructed".
Protesters from the UK and abroad are planning a week of demonstrations which will highlight what they claim are the links between climate change and aviation.
A "mass direct action" is scheduled for next Sunday and a website supporting the camp has promised acts of "civil disobedience".
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Organisers say that a "temporary eco-village" has been set up near the villages of Sipson and Harlington - between the M4 motorway and the airport's northern perimeter.
Protesters claim that the growth in air travel is a major factor in greenhouse gas emissions.
"Holding the camp at Heathrow aims to highlight the lunacy of the government's airport expansion plans," says a statement from campaigners.
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.
There are reports that the site, only 800 metres from BAA's Heathrow headquarters, was occupied by a group of protesters overnight.
A police spokesman said that about 150 people have set up camp at a sports ground belonging to Imperial College London - and that the protest was peaceful.
The camp will culminate in a day of protest
Airport operator, BAA, has warned against protests which could be a dangerous distraction at a time of heightened security fears.
Last week, BAA won a High Court ruling banning certain protesters from Heathrow - but the injunction does not prevent the setting up of the camp.
"With the current terrorism threat, keeping Heathrow safe and secure is a very serious business. Any action taken by the protestors that distracts us or the police from this task is irresponsible and unlawful," said Mark Bullock, BAA Heathrow's managing director.
"Around 1.5 million passengers are due to pass through Heathrow during the week of climate camp, many of them families on their summer holidays.
"It is our responsibility to ensure that we do everything we can to guarantee their safety and comfort during this very busy period."
The protesters have also been joined by a local resident, James Payne, who says that uncertainty about the future expansion of the airport had prevented him from selling his house.