The activists, dressed in white overalls and equipped with breathing masks, blocked access to the open cast Proschim mine close to the Polish border, according to an AFP journalist at the scene.
The group unfurled banners on excavation machinery, with slogans including “Keep it in the ground” and “Climate crime scene” as organisers said that the action would last all weekend.
“Each new tonne of coal is a tonne too many,” said the organisers of the protest, part of the “Break Free” campaign which was launched in several countries including the US, Canada and Brazil earlier this month to oppose fossil fuel use.
The “Break Free” campaign was the brainchild of several environmental lobby groups including Greenpeace and 350.org.
The targeted mine is operated by Swedish state-owned energy giant Vattenfall and produces 20 million tonnes of brown coal every year.
Vattenfall said in April that it had reached a deal to sell its German coal operations, which employ 8,000 people, as it moves away from activities blamed for climate change.
Vattenfall said it would sell its German brown coal, or lignite, business — open cast coal mines and two power plants close to the German-Polish border — to Czech operator EPH.
Coal operations represent about a tenth of Vattenfall's power production in Germany, where it is the third-largest energy supplier.