The Datteln 4 coal-fired plant is scheduled to come online next year despite the government pledging to phase out coal by 2038.
More than 100 anti-coal activists on Sunday stormed a planned German power plant to denounce the government's energy laws.
"Today we are occupying the Datteln 4 plant to make it clear to politicians and [international energy company] Uniper that we will not tolerate a new hard coal-fired power plant to go online in 2020," a statement from the Ende Gelände activist group behind the sit-in explained.
Police said they were attending the scene in "great numbers".
The group described the new coal law, passed py parliamentarians on Wednesday, as "a disaster" and called for "an immediate coal phase-out".
The law aims for coal to be removed from the country's energy mix by 2038 at the latest with the first plant closure scheduled for the end of the year.
However, Datteln 4, is expected to open as planned next year because it is considered less polluting than older plants.
Environment Minister Svenja Schulze hailed the law as "an important contribution to climate protection"
"It is an important international signal...We are demonstrating how an industrialised country can switch from coal-based electricity generation to renewable energies and, at the same time, create new economic prospects for coal regions," she added in a statement.
The government had, in December, unveiled a €54 billion package of measures to tackle climate change of which more than €45 million have been earmarked to go to the coal plants operators, the employees and regions most impacted by the coal phase-out.