Police told AFP that 14 people had entered the grounds of the Datteln coal station in North Rhine-Westphalia state, and some had chained themselves to machinery.
The plant has been met with several protests over the past couple months, with demonstrators also entering the grounds last week.
The power station is controversial because it is slated to come online just after Berlin agreed a long-term plan to exit coal-fired power by 2038 as part of a drive to reduce greenhouse gas emissions.
Activists argue the move to open a new plant flies in the face of Germany's ambitions to massively reduce carbon dioxide (CO2) output in the coming decades.
The plant at Datteln, reflected in a puddle. Photo: DPA
Eleven of them had climbed onto diggers or chained themselves to conveyor belts, the protesters said.
Supporters of the new power plant have argued that it will enable dirtier brown coal-fired stations to be shut down sooner.
But earlier this month, a study by Berlin economic think-tank DIW estimated the new plant will generate 40 million tonnes of additional CO2 emissions.