The Swedish 20-year-old has been participating in protests against the demolition of the village of Luetzerath, which is due to be cleared to make way for the expansion of a nearby coal mine.
A spokesperson for the local Aachen police force said Thunberg was part of a group of protesters that "stormed" towards the edge of the open-cast mine, described by officers as "steep and extremely dangerous." The activist was not arrested but was carried away and is now being held along with other protesters for identification.
The village of Luetzerath is owned by utility firm RWE and has been emptied ahead of its demolition to allow for the expansion of the nearby Garzweiler coal mine. Environmental activists oppose the expansion, arguing that bulldozing the village would result in significant greenhouse gas emissions from the mined fossil fuels.
The German government, however, says the expansion is necessary for energy security in the short term and that it is committed to phasing out coal use by 2030.