Hundreds of climate demonstrators set up a tent camp next to London's Heathrow airport on Monday and threatened "direct action" at the world's busiest air hub to protest against global warming.
Police with batons were on alert at the field where the camp is based, within one kilometre of the airport complex.
The campaigners plan a week of activities they say will culminate on Sunday in 24 hours of "direct action" in a bid to force the government to halt the planned expansion of the airport, which they argue will exacerbate climate change.
"The expansion of the airport just runs completely in the face of everything we know about climate change," said activist Tom James at the camp. "We are bringing totally unacceptable risks for future generations and people around the world."
Organisers said "direct action" could involve a mass picnic in Sipson, a town slated for destruction if a third runway is built, and targeting of the offices of airport operator BAA, owned by Spain's Ferrovial.
But they said the airport's perimeter fence would not be breached out of concerns for public safety.
Banners urging a halt to airport expansion fluttered over the marquees and tents were being erected in the field as supplies of fruit and vegetables were brought in. Protesters ranged in age from pensioners to teenagers and children.
The protest comes at the height of the holiday season at an airport that handles nearly 70 million passengers a year.
Local resident Tom Creeden said the anti-expansion campaigners were fighting a losing battle: "It is progress and it is needed. You have got to have a bigger airport and it might as well be here as anywhere else."
BUSIEST TIME OF YEAR
Scientists say air transport contributes heavily to global warming, noting that the carbon dioxide and water vapour emitted at altitude are four times more potent than at sea level.
Police said around 250 protesters had arrived at the camp, just to the north of the airport and directly in line with the site of the proposed third runway. Up to 1,500 campaigners are expected to join the camp over the week.
Police said extra officers had been drafted in.
Commander Jo Kaye, who is coordinating the operation, said police were prepared should the demonstration turn unruly.
"There is a wide range of groups here, some who say they won't disturb passengers at all," he said. "There will be some people who arrive to demonstrate who are perfectly prepared to break the law."
But Kaye added that counter-terrorism operations at Heathrow were unaffected.
"The picture doesn't change because there's a climate camp nearby," he said. "We're very, very used to policing this sort of thing."
BAA was concerned about the potential for chaos because August is the busiest month for Heathrow's four terminals.
"Around 1.5 million passengers are due to pass through Heathrow during the week of the climate camp, many of them families on their summer holidays," a BAA spokesman said.
A spokeswoman for Prime Minister Gordon Brown said peaceful protest was a legitimate right but that any action to disrupt the operation of Heathrow would be "unacceptable".
