Environmental protesters occupying the Hambacher Forst forest threw rocks at police and loggers Monday, as clearing work began to expand one of Germany's largest open-pit coal mines. The clashes provoked authorities to respond with pepper spray.

One protestor, who goes by the name Oaktown on Twitter, posted footage of police using pepper spray to disperse protestors.

Local media reported that three demonstrators managed to break past police barricades into one of the areas marked for deforestation, before being detained.

Read more: Environmental activists storm open-pit coal mine ahead of climate talks

The Hambach mine, located between the German cities of Aachen and Cologne, near the border to the Netherlands, is already one of Germany's largest lignite mines and one of the largest open-pit operations in the world. Each year, it produces some 40 million tons of lignite — a brown, low-grade coal considered to be one of the most polluting fossil fuels.

The Hambach mine has also been the site of numerous protests calling for the German government to end its use of coal. Germany's sudden turn away from nuclear power after the Fukushima meltdown in Japan forced Angela Merkel's government to increase coal power production to compensate.

Activists have occupied parts of Hambacher Forst forest since 2012.

Read more: #DailyDrone: Hambach surface mine

While authorities had already cleared some of the pitched tents and treehouses, parts of the forest set for clearing still remain occupied. Demonstrators erected large barricades to prevent police and workers from accessing those areas.

Police also said that they had cleared boards with nails piercing through, designed to damage vehicles' tires, along one of the roads leading to the forest.

RWE gets legal permission to begin deforestation

On Friday, a Cologne court ruled against claims made by the environmental body BUND that the area marked for deforestation was protected by environmental legislation. The ruling prompted German energy giant, RWE, which runs the mine, to begin felling operations.

Earlier on Monday, Aachen police put out a statement saying it was expecting major resistance against the deforestation plans, adding that a large number of officers had been deployed to protect RWE employees and facilities in the Hambacher Forst.

RWE's plans to expand the Hambach mine have also prompted political resistance from Berlin, with Green party co-chair Simone Peter tweeting: "Although the Cologne Hambach is not yet legally binding, RWE is already creating facts [on the ground]."

Read more: How far is Germany from a complete coal exit?

RWE spokesman Guido Steffen responded saying that all clearing operations were legally sound and in line with the Cologne court's ruling.

Protests set to continue long-term

With protests expected to continue, RWE said it was impossible to estimate when the logging would be completed and the coal mine opened. "Let us not fool ourselves; the difficulties will come when we have to enter the occupied forest. There is no telling what kind of resistance there will be," said Steffen.

Aachen police have said they were preparing for further protests on Tuesday.

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dm/msh (AFP, dpa)