Dozens of environmental activists broke into a secure area of a Stansted Airport near London Monday, chaining themselves to each other and barricading themselves behind fencing in a protest against air-traffic pollution.

Protest Group Plane Stupid, known for its attention-grabbing stunts, said activists stormed the airport before dawn and staged their protest near the taxi area where planes travel between takeoff and landing.

The activists are opposed to a possible expansion of the airport.

Police said that 57 activists were arrested, most on suspicion of trespassing.

Dozens of flights at the airport, about 60 kilometres northeast of central London, were cancelled as police worked to clear the demonstrators, who had pulled security fencing around them as a barricade.

Inside the airport, thousands of passengers were trying to get information about their flights as budget airline Ryanair pointed the finger at Stansted's operator, BAA Airports Ltd.

"It is unacceptable that the travel plans of thousands of passengers have been disrupted because BAA Stansted security have failed to remove a number of protesters," the airline said, adding that 52 of its flights had been cancelled.

Stansted's operator said it could not police every inch of the airport all the time.

"You should bear in mind, of course, that the runway is about 2,000 acres [819 hectares] in size," BAA commercial director Nick Barton told BBC radio. "It is an enormous area, and we don't intend to run an airport as a fortress."

Plane Stupid is fiercely opposed to the expansion of air travel, something it claims contributes disproportionately to global warming.

The group posted video and pictures on its website that appeared to show demonstrators in fluorescent vests emblazoned with the words "Please DO something" facing off with police in vehicles. Video on the site showed what the group said was a snow plow ramming the demonstrators' barricade.

Plane Stupid has a history of dodging strict security while ensuring maximum media attention.

In July, one of the group's activists unsuccessfully tried to glue himself to British Prime Minister Gordon Brown at an awards ceremony. Earlier this year, demonstrators climbed onto the roof of Britain's Houses of Parliament and unfurled a banner protesting plans to build a third runway at London's Heathrow Airport.

Last year, when environmental activists descended on a site near Heathrow to protest that airport's proposed expansion, authorities got an injunction barring Plane Stupid from the area.

Group members instead staged a sit-in on a barge transporting a newly built wing of an Airbus A380 down a river.