Six men have pleaded not guilty in Abuja to treason and terrorism charges over an alleged 2025 coup attempt.
Nigerian media report first prosecutions as authorities pursue case over a purported plan to overthrow the government.
Government initially denied the coup attempt but later announced in January it had foiled the alleged plot.
Successful coup would have ended twenty-five years of democracy in Africa’s most populous nation Nigeria.
The court case involves six civilians including retired Major General Mohammed Ibrahim Gana among the accused.
Others include retired Captain Erasmus Ochegobia Victor, Ahmed Ibrahim, Zekeri Umoru, Bukar Kashim Goni, Abdulkadir Sani.
Military officers are also expected to face separate trials that have not yet commenced.
At the Federal High Court in Abuja the defendants entered not guilty pleas during their initial appearance.
Security forces restricted journalists including AFP, BBC, Deutsche Welle and Reuters from entering the courtroom.
Lawyers for prosecution and defence declined to speak to journalists gathered outside the court.
Tina Umoru said she cannot reach her husband whose phone is switched off as children ask where he is.
A defence lawyer alleged some defendants were held by the Defence Intelligence Agency without access to their legal counsel.
Justice Joyce Abdulmalik ordered custody under the State Security Service ensuring access to lawyers according to reports.
Charges allege conspiracy to levy war against state in an attempt to overawe the President of Nigeria.
They are accused of knowing Colonel Mohammed Alhassan Ma’aji and others intended treason but failed to alert authorities.
Ma’aji has been described in Nigerian press reports as the alleged mastermind of the attempted coup plot.
The documents also accuse the defendants of terrorism-related support and indirect assistance to the alleged conspirators.
Former oil minister Timipre Sylva is named as still at large, though he denies connection to the plot.