Stansted Airport closed its only runway for about three hours, re-opening around 1330 IST after police arrested 57 people who had breached the perimeter fence using bolt-cutters under cover of darkness.

The protest prompted budget Irish airline Ryanair, which cancelled more than 50 flights, to call for an urgent review of security at the airport which serves as its main hub.

The Plane Stupid protest group was demonstrating against the British government's decision to allow the airport to build a second runway.

One activist, Lily Kember, 21, said protesters entered the airport, erected fencing around themselves and were able to sit on a site near where airplanes taxi before taking off and landing.

"Being arrested is a terrifying prospect, but not nearly as terrifying as the threat of climate change," she said.

Stansted's commercial manager Nick Barton said the activists had driven up to the airport in an "old fire engine".

"It probably didn't look out of place in all honesty as there are a number of emergency services vehicles around the site," he said.

"They managed to cut through the fence. As soon as they were through, they ran through the gap they had created and brought the fence panels to barricade themselves in."

He said airport security staff had prevented the activists from reaching the runway.

"Our site is 2,000 acres in size and the perimeter is enormous," he said.