Amnesty International on Wednesday said it had “huge concerns” about the mass trials of Boko Haram suspects, with press and the public banned from the hearings on security grounds.
Proceedings began this week at four civilian courts set up at a military base in Kainji, in central Niger state, with 1,669 suspects due to appear before judges.
The government has said 651 others held in the northeastern city of Maiduguri will be tried once the cases in Kainji are finished.
Amnesty’s Nigeria’s director, Osai Ojigho, said the trials “should provide a much-needed opportunity to deliver justice” for those affected by the long-running conflict.
“However, the fact the trials are taking place behind closed doors, with no access for the media or the public, raises huge concerns,” he added in a statement.
“Public hearings are crucial for protecting an individual’s right to a fair trial and due process.”
Three hundred suspects were officially remanded in custody for 90 days on Tuesday while one defendant was transferred for trial elsewhere.
Nigeria has promised that every defendant will have legal representation.
Rights groups have repeatedly accused Nigeria’s military of arbitrarily arresting thousands of civilians since the start of the Islamist insurgency in 2009.
Detainees have in many cases been held for years in unsanitary, overcrowded conditions without access to lawyers or ever having appeared in court.
Hundreds of people, including young children picked up with their parents, have been released without charge.
Ojigho said of those at Kainji: “In instances where no prima facie case has been established… detainees should be immediately released.”