The group of far-right and ex-military figures are said to have prepared for a "Day X" to storm the Reichstag parliament building and seize power.
A man named as Heinrich XIII, from an old aristocratic family, is alleged to have been central to their plans.
According to federal prosecutors, he is one of two alleged ringleaders among those arrested across 11 German states.
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 and antisemitic conspiracy theories.
They also refuse to recognise the modern German state.
Other suspects came from the QAnon movement who believe their country is in the hands of a mythical "deep state" involving secret powers pulling the political strings.
Interior Minister Nancy Faeser assured Germans that authorities would respond with the full force of the law "against the enemies of democracy".
An estimated 50 men and women are said to have been part of the group, which allegedly plotted to overthrow the republic and replace it with a new state modelled on the Germany of 1871 - an empire called the Second Reich.
"We don't yet have a name for this group," said a spokesperson for the federal prosecutor's office. The interior minister said it was apparently made up of an organisation "council" and a military arm.
Wednesday's dawn raids are being described as one of the biggest anti-extremism operations in modern German history.
Three thousand officers took part in 150 operations in 11 of Germany's 16 states, with two people arrested in Austria and Italy.
Almost half of arrests took place in southern states of Baden-Württemberg and Bavaria. More than one in five Reichsbürger are thought to be based in the south-western state of Baden-Württemberg alone.
Justice Minister Marco Buschmann tweeted that a suspected "armed attack on constitutional bodies was planned". Faeser said later that the investigation would peer into the "abyss of a terrorist threat from the Reichsbürger scene".
German special police forces detain Heinrich XIII Prinz Reuss after searching a house in Frankfurt / Main, western Germany, on 7 December, 2022. Photo: Boris Roessler / dpa / AFP