LONDON - Environmental protesters cut through the perimeter fence at Britain's third-largest airport yesterday and occupied the tarmac, forcing the cancellation of more than 50 flights and raising concerns about airport security.

Two waves of protesters arrived at Stansted Airport before dawn. The first used an old fire truck to provide cover as they sliced through the perimeter fence with bolt cutters. A second group ran through the gap. The protesters then chained themselves together and erected a barrier around themselves.

It took nearly five hours to clear the tarmac. Police arrested 57 people.

"If they can do it, I'm sure there would be nothing to stop terrorists," said Terry Purton, whose flight to Bratislava, Slovakia, was delayed.

The action - meant to protest a possible second runway at Stansted and escalating carbon emissions - caused 56 cancellations and extensive delays. The British government is also considering adding another runway to Heathrow Airport, already Europe's busiest.

Stansted authorities said the runway at the airport about 30 miles north of London was never penetrated by the protesters. Security monitors the airport perimeter with cameras and patrols.

"We are dealing with a security breach, which has caused the runway to be closed and flights canceled, but at no time were any passengers or planes in danger," said Nick Barton, commercial director of BAA at Stansted, the company that runs the airport. "We knew they were breaking in as soon as they started cutting the fence."

Protesters have staged demonstrations recently at airports in London, Athens and Thailand - targets that offer global publicity to highlight an array of grievances.

"Airports are visible and vulnerable global trading centers," said Philip Butterworth-Hayes, founding editor of Jane's Airport Review. "They're more than that: They're symbols of nationality as well."