The trial of nine suspected members of a far-right group headed by German aristocrat Prince Heinrich XIII Reuß on charges of terrorism and high treason started in Stuttgart on Monday.
Due to the large crowd outside the court building, the trial started more than an hour later than planned. Prince Reuß himself is to be tried later in another trial.
The nine suspected members of the Reich Citizens movement, known in German as Reichsbürger, a group that rejects the legitimacy of the modern German state, are accused of membership of a terrorist organization and "preparation of a treasonous undertaking."
One faces an additional charge of attempted murder.
The proceedings in Stuttgart are the first of three mammoth trials against the group around Prince Reuß and one of the largest state security proceedings in Germany's history.
Interior Minister Nancy Faeser told broadcaster ZDF on Monday morning: "It shows the strength of our constitutional state that the largest terrorist network of 'Reich Citizens' to date is now on trial and must answer for its ... militant plans to overthrow the state."