The Hambach mine has become a cause celebre for environmentalists amid plans to clear part of neighboring Hambach Forest to make way for an expansion. Earlier this month, a German court blocked the felling of the forest near Cologne, saying it needs time to examine whether the woodland deserves protected status.
The dpa news agency reported that activists said more than 5,000 people were heading toward the mine on Saturday. Police said about 40 protesters occupied an excavator, with eight of them climbing onto the machine.
Separately, employees of the energy company RWE were demonstrating for their jobs to be preserved.