A leader of a Niger Delta oil militant group has been formally charged with treason in a closed court session.
Henry Okah, a key leader of the Movement for the Emancipation of the Niger Delta (Mend), faces a death sentence if convicted.
The court in the northern city of Jos was heavily guarded by security officers during the hearing.
It is the first time Mr Okah has appeared in court since he was secretly extradited from Angola in February.
His lawyers have been barred from talking about the case by the court who also banned journalists from attending.
"Every coup plotter in Nigeria was tried in the open... This is a case of gun-running and it does not warrant this kind of trial," defence lawyer Femi Falana said.
Mend has said it will restart its campaign of violence in the Niger Delta if Mr Okah does not receive a fair trial.
He has so far been kept in a secret location and prevented from seeing his lawyers for weeks.
Oil production has been slashed by a fifth in recent years because of attacks on pipelines.
Mend also kidnapped foreign oil workers for ransom.
The organisation say they are fighting for a better deal for people who live in the oil producing region.
Mr Okah and Edward Atatah, a captain in the Nigerian merchant navy, were arrested in September in Luanda.
Interpol says they were buying arms and explosives, but they claim they were buying a second-hand trawler for a marine engineering business.
Mend came to prominence in 2006 after the arrest of another militant leader, Mujahid Dokubo Asari.Mr Asari was released last year just before elections but his militant group lost its influence in the Delta after they were accused of selling out to the government.