Police officers have mobilised against fierce resistance since the middle of last week to clear the demonstrators from Hambach Forest near the border with Belgium and the Netherlands.
Up to 4,000 police have been on the scene to remove 60 treehouses, some as high as 25 metres (80 feet) off the ground, housing dozens of protesters.
The occupation began in 2012 and had until now been quietly tolerated. But local authorities have now ordered the woods to be cleared immediately, citing fire hazards.
The activists, who are protesting the expansion of energy giant RWE's open-pit lignite mine, one of Europe's largest, had called for a mass mobilisation by supporters to prevent the forest from being cleared.
The battle has intensified since RWE announced plans to clear half of the forest's remaining 200 hectares (500 acres) from mid-October. RWE owns the forest and is legally allowed to cut down trees to access the brown coal, or lignite, in the ground during the annual logging season.
It says the clearing is necessary to ensure energy supply, including of nearby power plants in Germany's most populous state, North Rhine-Westphalia.
Activists oppose the use of the cheap but polluting fuel, and say the forest is home to protected species such as Bechstein's bat and century-old beech and oak trees.
Their protest has taken on fresh urgency as Germany is charting an exit from coal energy to combat climate change.
A government-appointed coal committee is due to announce an end date for the industry by the end of the year.
Germany has massively expanded renewable energy in recent years as part of a transition away from fossil fuels.
But the country remains heavily reliant on coal, partly to offset Chancellor Angela Merkel's 2011 decision to phase out nuclear power by 2022. The government admitted in June that it will miss a 2020 target for
reducing carbon dioxide emissions.
Rather than cutting emissions of greenhouse gases by 40 percent by 2020 compared with 1990 levels, Europe's top economy expects to come in at 32 percent.