The Nigerian armed forces on Friday swore in the justices for the military court that will try three dozen officers accused of a botched coup plot, the country's Defence Headquarters said.

The government had initially denied the existence of the alleged attempt late last year, before announcing in January that it had foiled a plan to overthrow the government.

If successful, it would have brought an end to a quarter-century of democracy in Africa's most populous country.

The Defence Headquarters "inaugurated a General Court Martial to try 36 serving military personnel over alleged mutiny, aimed at overthrowing the government of the day", spokesman Major General Samaila Uba said in a statement.

The statement did not specify a timeline for the trials but said that "proceedings of the court will be conducted with the highest standards of fairness".

The government-owned News Agency of Nigeria (NAN) reported that the accused officers were brought to a military facility in the capital Abuja Friday morning for the inauguration of the court martial, though it is unclear when prosecutions will get underway in earnest.

Friday's inauguration was held behind closed doors, NAN reported.

Civilians charged

Separately, six civilians, including a retired major general, were charged over the alleged coup plot at the Abuja Federal High Court earlier this week.

They pleaded not guilty.

Those charges named retired major general Mohammed Ibrahim Gana, retired captain Erasmus Ochegobia Victor, Ahmed Ibrahim, Zekeri Umoru, Bukar Kashim Goni and Abdulkadir Sani.

They are accused of conspiring "with one another to levy war against the State to overawe the President of the Federal Republic", according to the charging documents, seen by AFP.

The six were also accused of knowing that Colonel Mohammed Alhassan Ma'aji "and others intended to commit treason" and did not alert authorities.

Ma'aji has been named in previous Nigerian press reports as the "mastermind" of the coup.

The names of the officers being tried in the court martial were not released Friday.

Last year, it was announced that 16 officers were arrested over "issues of indiscipline".

Despite official denials at the time, sources across the Nigerian government and military told AFP the arrests were linked to a foiled coup plot.

In January, in a reversal, the military announced it would be trying "a number of the officers with allegations of plotting to overthrow the government".

Nigeria has seen several military takeovers in its history and spent much of the 20th century under junta rule after its independence from Britain in 1960.

The west African nation transitioned to civilian rule in 1999 and has had a democratic government since.

Shortly after denying the alleged coup plot, President Bola Tinubu reshuffled the country's top military brass.