The Reichsbuerger don't recognise the legitimacy of the modern German republic and generally believe in the continued existence of the pre-war German Reich.
A mixed bag of conspiracy theorists, neo-Nazis and gun enthusiasts who reject the modern democratic state, the fragmented movement is growing more radical at a time when right-wing extremism in general is on the rise.
Several "Reichsbuerger" have opened fire on police or hoarded weapons and explosives.
On Wednesday, 200 police raided 12 locations nationwide and detained two suspects, accusing them of plotting "armed attacks against police officers as representatives of the state, asylum seekers and members of the Jewish community".
What do we know about the Reichsbuerger?
The Reichsbuerger refuse to recognise the legitimacy of the modern German republic and generally believe in the continued existence of the pre-war German Reich, while some idolise Nazi Germany.
Some see modern Germany as a US colony and argue that the Reich's 1919 Weimar Constitution and its formers borders which extend far into present-day Poland remain valid.
Several groups have declared their own mini-states, such as the "Exile Government of the German Reich" or the "Free State of Prussia", made flags and issued their own identity papers, currencies and postage stamps.
They typically deny the legitimacy of police and other state institutions and refuse to pay taxes, social service contributions and fines, which brings them into conflict with authorities.
Their members have often been dismissed as eccentrics, oddballs and trolls known for swamping local municipalities and courts with letters, formal complaints, motions and objections.
The movement is characterised by an extreme right-wing, historically revisionist and often ethno-nationalist ideology.
Several groups are known to have direct links to illegal neo-Nazi organisations and are under surveillance by German state and federal intelligence agencies.
Berlin's security service describes Reichsbuerger as following an "ideological mix of conspiracy theories, anti-Semitic and anti-democratic views".
The anti-fascist Amadeu Antonio Foundation warns that although it may be tempting to dismiss Reichsbuerger as cranks, "behind the masquerade of conspiracy theories, esotericism and government game-playing lies a hardcore right-wing extremism and anti-humanist ideology".
Around since the 1980s, with links to like-minded groups in the United States, the movement was long estimated to number in the hundreds -- but it has grown exponentially in the internet age, especially in the past few years.
This week domestic intelligence service chief Hans-Georg Maassen said the "Reichsbuerger scene" of activists and sympathisers was now thought to number about 10,000.
Of these, 500 to 600 were known right-wing extremists, Maassen told the DPA news agency.
Most known followers are middle-aged white males. They exist nationwide, but most high-profile cases have been reported in Germany's formerly communist east and the southern state of Bavaria.
Maassen said his service worried about the movement's "considerable propensity for violence and increased aggressiveness".
In an attack last August, a 41-year-old Reichsbuerger and one-time "Mister Germany" pageant winner opened fire on police carrying out an eviction order at his house in the eastern state of Saxony-Anhalt.
The gunman was seriously wounded and three police officers suffered light injuries.
Another Reichsbuerger is accused of shooting dead a police officer and wounding three others during a raid to confiscate his hunting weapons in the town of Georgensgmuend, Bavaria, in October.
After the killing, it emerged that some German state police forces were sweeping their own ranks to search for possible Reichsbuerger and sympathisers in uniform.
"Some Reichsbuerger display signs of mental health issues," an intelligence service official from the eastern state of Brandenburg, Heiko Homburg, told Bild newspaper this week.
"If they have access to weapons, they pose an extreme threat."