LONDON (AP) -- Environmental protesters cut through the perimeter fence at Britain's third-largest airport Monday and occupied the tarmac, forcing the cancellation of more than 50 flights and raising questions over the vulnerability of airports.
Two waves of protesters arrived at Stansted Airport before dawn. The first group 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, 61, a passenger whose flight to Bratislava 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 - which is situated on about 2,000 acres (3.12 square miles) north of London - was never penetrated by the activists. Security monitors the airport perimeter with cameras and occasional 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, the BAA commercial director 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 (but) they're more than that: they're symbols of nationality as well," said Philip Butterworth-Hayes, the founding editor Jane's Airport Review. "There is a lot of drama at airports."
Thai protesters occupied Bangkok's two airports in a weeklong siege that ended last week, leaving more than 300,000 travelers stranded and dealing a severe blow to the country's tourism industry.
In Greece, Olympic Airlines employees upset over plans to privatize the state-owned airline repeatedly walked onto the runway at Athens International Airport, occasionally disrupting flights.
Plane Stupid - the group behind the Stansted protest - has promised more protests if the third runway at Heathrow is approved. A decision is expected by next year.
The group is known for breaching tight security in attention-grabbing stunts.
In July, a member tried to glue himself to British Prime Minister Gordon Brown at an awards ceremony. In February, activists managed to sneak on to the roof of Britain's Houses of Parliament.
Philip Baum, the editor of Aviation Security International, said there is little that can be done to completely secure an airport's perimeter.
"That's what you saw last week in Bangkok; they didn't need to cut through any perimeter fence at all," said Baum, with the London-based publication. "The reality is, unless you're going to have armed guards around the perimeter in the airport, it's difficult to stop."
Britain's environmentally conscious public and ambitious emissions-cutting goals have often clashed with its desire to build up the country's air infrastructure.
It's worrying," said Anne Nordbye, 35, from Oslo, Norway, whose flight was delayed. "How did they do it?"