London's Stansted airport reopened to traffic Monday, Dec. 8, after being shut down for several hours by environmental protestors, an airport spokesman said.

The early morning action by some 50 protestors from the group Plane Stupid had closed the airport's single runway, paralyzing all traffic.

A total of 21 flights were canceled, as long lines formed at check-in counters and many passengers chose to return home or seek hotel accommodation, reports said.

Up to 40 protestors managed to break through a tall security fence at around 3 am local time on Monday, were arrested, police said.

Stansted, which is some 50 kilometers (30 miles) north-east of London, is the region's third-biggest airport after Gatwick and Heathrow and used mainly by budget airlines, including Ryanair, for their European routes.

The Plane Stupid group said the protest was to highlight the "hypocrisy" between government climate change goals and reality.

"We're here because our parents' generation has failed us and its now down to young people to stop climate change by whatever peaceful means we have left," a statement posted on the group's Web site said.

"Being arrested is a terrifying prospect, but not nearly as terrifying as the threat of climate change," one female protestor told the BBC Monday.

The protest is over the government's decision to allow the expansion of Stansted with a second runway, a plan that has met widespread opposition from residents in nearby towns and villages.