Several of those targeted were considered close to the so-called "Reichsbuerger", or Citizens of the Reich.
Several of those targeted were considered close to the so-called "Reichsbuerger", or Citizens of the Reich -- a shadowy extremist movement whose members reject the legitimacy of the German republic.
The chief suspect detained was a 62-year-old self-proclaimed "Celtic druid", who had urged violence against Muslims and Jews in online postings, news agency DPA and other media reported.
Police swooped on 12 homes and other sites in six states "as part of a federal investigation on suspicion of forming a right-wing extremist organisation", the prosecutor's office said.
The officers confiscated weapons, munitions and explosives, a spokeswoman added later.
Six suspects, "connected primarily via social media", are accused of founding the group "and in early 2016 beginning plans for armed attacks against police officers as representatives of the state, asylum seekers and members of the Jewish community".
Another suspect was believed to have offered assistance to the group including by acquiring weapons.
"The aim of today's raids is to gather evidence of the formation of a group as well as suspected crimes and potential material for use in those crimes," the prosecutors' statement said.
"There is not as yet any evidence of specific attack plans."
Around 200 police officers took part in the coordinated raids.
DPA said the group included members of the "Reichsbuerger", which is blamed for shooting dead one police officer and wounding three others during a raid in the southern town of Georgensgmuend in October.
In another attack last August, a member of the group -- a former "Mister Germany" pageant winner -- opened fire on police carrying out an eviction order at his house in the eastern state of Saxony-Anhalt.
The 41-year-old gunman was seriously wounded and three police officers suffered light injuries.
Most Reichsbuerger believe in the continued existence of the German empire or "Reich", and many members refuse to pay taxes, social security contributions and fines owed to the state.
The group is thought to overlap with neo-Nazis, extreme adherents of conspiracy theories and other esoteric beliefs.
According to Germany's domestic intelligence service, the Reichsbuerger movement has some 10,000 members.
"The Reichsbuerger movement.... continues to win new followers," domestic security chief Hans-Georg Maassen told DPA, voicing concern about the "increased aggressiveness" of its followers.