BERLIN — The police started evicting protesters from treehouses on Friday in what remains of a formerly vast forest in Western Germany, moving cranes and backhoes into a strip of ancient woodland where activists have lived for years on the edge of a growing open-pit mine.

Pushing their way into the heart of a forest that has existed for 12,000 years, where some 60 treehouses have been built over the past six years, thousands of police officers began cutting trees to reach a central community called “Oaktown.” They tore down at least one treehouse and detained more than a dozen protesters, on the ground and in the trees, the police said.