London  - A protest by environmental protestors Monday closed down London's Stansted airport, causing severe delays for passengers due to arrive and depart from the city's third-biggest airport used mainly by budget airlines on European routes.

More than 50 supporters of Plane Stupid, a climate action group, blocked the single runway at Stansted, north-east of London, as passenger queues grew longer and planes were diverted.

Four activists were arrested when the group broke through the security fencing early Monday. They camped on the runway and used the wire fencing for protection, as police attempted to move protestors aside.

BAA, operator of Stansted Airport, said the runway was closed for "safety and security reasons" and warned passengers to expect delays.

"Due to this event the runway is closed and will continue to be so until further notice. The advice to passengers is to contact their airlines directly who can advise them but we are expecting delays."

The Society of British Aerospace Companies criticized the action.

"These protestors are ignoring the reality around aviation and climate change. Flight numbers have increased at Stansted but noise nuisance around the airport has actually decreased and aircraft are 70 per cent more fuel efficient than they were 50 years ago," spokesman Matthew Knowles said.

But the protestors said they wanted 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 website said.

The group planned to maintain the blockage as long as possible to prevent the emission of greenhouse gases, Plane Stupid 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. (dpa)