Hundreds of police officers descended on the area in the early morning, after local authorities ordered the Hambach Forest in western Germany to be cleared immediately, citing fire hazards.
Dozens of protesters are holed up in some 60 treehouses, some as high as 25 metres off the ground. The occupation began in 2012 and their presence had until now been quietly tolerated.
But the state premier of North Rhine-Westphalia, Armin Laschet, told local broadcaster WDR late Wednesday that this was “an illegally occupied area” and accused the protesters of being violent.
The activists, who are protesting the expansion of energy giant RWE's massive open-pit lignite mine, one of Europe's largest, have vowed to peacefully resist the evacuation.
“For many of us this is home. Some of us have lived here for years,” a tree-dweller who gave his name only as Freddy told DPA news agency.
A spokesman for the city of Kerpen said the forest dwellers had been told by megaphone to clear the site.
Activists gather to stop police from clearing out the treehouses in the Hambach Forest. Photo: DPA
“If they don't come down voluntarily, we will clear the treehouses with the help of police,” he told DPA. As well as a huge police presence, a DPA reporter described seeing heavy clearing equipment and water cannons at the scene.
The David versus Goliath battle in the forest has been intensifying for days after RWE announced its plans to clear half of the forest's remaining 200 hectares from mid-October.
A police officer fired a warning shot in the forest Wednesday, Aachen police said in a statement, after “several masked people once again attacked police by throwing stones”.
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.
But activists oppose the use of the cheap but polluting fuel, and say the forest is home to protected species like 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.
Police begin entering the forest, under one of the activist's tree platforms. Photo: DPA
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 its “energy 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 manage reductions of just 32 percent.
The activists, who are protesting the expansion of energy giant RWE's giant open-pit lignite mine, one of Europe's largest, have called for a mass mobilization by supporters in coming days.
“The destruction of Hambach Forest is intolerable,” said Jan Puetz of Aktion Unterholz in a joint statement with other activist groups.
“Starting this weekend, we will prevent the evacuation and the forest cutting by police and RWE with actions of mass civil disobedience”.
State police warned on Twitter that the height at which the demonstrators were hunkering down “poses a danger to EVERYONE. We don't want people to get hurt.”
The police intervention comes a day after an officer fired a warning shot in the forest, Aachen police said in a statement, after “several masked people once again attacked police by throwing stones”.
A live video streamed on the Spiegel Online website showed officers in a cherrypicker sawing off branches and cutting ropes to gain access to a tree platform occupied by a young couple, who were seen kissing before being separated.
A large rescue air cushion was placed on the ground.
While the bearded man went peacefully, the bare-foot woman clung to a tree trunk before being dragged into the cherrypicker lifting platform by two officers.
Police also peacefully broke up a sit-in by a group of demonstrators blocking the path to the treehouses and removed makeshift barriers, according to DPA news agency.
As well as a huge police presence, heavy clearing equipment and water cannons could be seen at the site.