More climate change campaigners are set to join protesters who have already set up a camp outside Heathrow airport
Up to 3,000 people were expected to pitch their tents on Tuesday for a week-long protest against plans to expand the airport.
But about 150 campaigners began building the so-called Camp for Climate Action on Sunday.
In response, airport operator BAA warned it would not allow passengers to be "harassed or obstructed".
Protesters from the UK and abroad are expected to highlight their claims that the growth in air travel is a major factor in greenhouse gas emissions.
'Direct action'
The camp is near the villages of Sipson and Harlington, between the M4 motorway and the airport's northern perimeter.
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.
A day of "mass direct action" is scheduled for Sunday and a website supporting the camp has promised acts of "civil disobedience".
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.
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.
A police spokesman said that about 150 people had set up camp at a sports ground belonging to Imperial College London - and that the protest was peaceful
.
Injunction
Around 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 will not affect day-to-day counter terrorist operation at Heathrow.
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," said Mark Bullock, BAA Heathrow's managing director.
"Any action taken by the protesters that distracts us or the police from this task is irresponsible and unlawful."
First published on the bbc news site|