Environmental activists have stopped flights at Essex's Stansted airport after breaking through to the runway, police and airport officials say.

"The runway has been closed," a spokeswoman for Stansted airport, which mostly serves short-haul European flights, said.

"All inbound aeroplanes have been held at their points of departure, so there have been no diversions," she added.

Police said the protesters had locked themselves to fencing and that 16 had been arrested so far in an ongoing operation.

"We would hope to get the airport fully functional by 8:00am (local time)," Stansted divisional police commander Ian Gruneberg told BBC radio, which said about 50 protesters had broken on to the runway.

Climate action group Plane Stupid said on its website it was behind the protest.

"The group intends to maintain its blockade for as long as possible, preventing the release of thousands of tonnes of greenhouse gas emissions into the atmosphere," it said.

"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."

Stansted is operated by BAA, a unit of Spanish infrastructure group Ferrovial and is a major base for budget carrier Ryanair among other airlines.

- Reuters