A far-right group is suspected of planning to storm parliament and install an aristocrat as leader.
Twenty-five people have been arrested in raids across Germany on suspicion of plotting to overthrow the government.
German reports say the group of far-right and ex-military figures planned to storm the parliament building, the Reichstag, and seize power.
A German man referred to as a prince called Heinrich XIII, 71, is alleged to have been central to their plans.
Among those arrested in 11 German states were two alleged ringleaders according to federal prosecutors.
The plotters are said to include members of the extremist Reichsbürger (Citizens of the Reich) movement, which has long been in the sights of German police over violent attacks and racist conspiracy theories. They also refuse to recognise the modern German state.
An estimated 50 men and women are alleged to have been part of the group who are said to have plotted to overthrow the republic and replace it with a new state modelled on the Germany of 1871.
“We don’t yet have a name for this group,” said a spokeswoman for the federal prosecutor’s office.