Berlin (CNN) Activists in a forest in western Germany are resisting attempts by police to remove tree houses that environmentalists have occupied for years and to make way for mining company RWE to raze further sections of the forest.

RWE runs an open-pit coal mine near the Hambach Forest in the state of North Rhine-Westphalia.

The operation launched by police there Thursday morinng marks a pivotal moment in the country's battle between green energy and coal-powered plants, which produce almost 40% of Germany's electricity, according to Eurostat

The 12,000-year-old Hambach Forest, which is owned by RWE, has shrunk to less than 10% of its original size since the company began razing sections of it four decades ago, according to estimates by RWE and environmental activists.

Following rumors that clearances were imminent, hundreds of police officers, many in riot gear, entered Hambach Forest on Thursday morning, clearing blockades erected by the occupants and demanding they leave their treetop dwellings.