Climate campaigner Greta Thunberg has been freed after she was detained by police when she tried to jump into a coal mine during a protest in Germany.

The abandoned hamlet of Luetzerath is to be bulldozed to make way for a coal mine expansion.

Activists have been attempting to stop the demolition in a simmering stand-off that highlights tensions over Germany’s climate policy.

Ms Thunberg had been detained while protesting at the open cast coal mine of Garzweiler 2 on Tuesday, some nine kilometres from the village of Luetzerath, after police warned activists they would be removed by force if they did not move away from the edge of the mine.

The controversial mine expansion comes as Germany has promised to phase out coal-fired power by 2030.

The village is owned by energy company RWE and the company has said the coal underneath is needed as early as this winter, the BBC reported.

A coal mine looms behind the village of Lützerath. Photo: Getty

Ms Thunberg was detained while sitting with a group of protesters near the edge of the mine.

She was carried away by three policemen and held by one arm at a spot further away from the edge.

Footage shows police warning the group that “we are going to use force to bring you to the identity check, so please cooperate”.

Police officers carry Swedish climate activist Greta Thunberg. Photo: Getty

Later Ms Thunberg, who joined the protesters on Friday, was seen sitting alone in a large police bus after having been detained, a Reuters witness said.

“Greta Thunberg was part of a group of activists who rushed towards the ledge,” a police spokesperson said.

“However, she was then stopped and carried by us with this group out of the immediate danger area to establish their identity.”

It was not yet clear what would happen to Ms Thunberg or the group she was detained with, or whether the activist who jumped into the mine was injured, the spokesperson said.