- To date, just 13 people have been put on trial and only nine convicted for their links to the Islamist insurgency.
- President Muhammadu Buhari has promised to look into repeated accusations of human rights violations.
More than 2,300 suspected members of the jihadist group Boko Haram were expected to appear in court in Nigeria from Monday in unprecedented mass trials to be held behind closed doors.
The defendants have all been picked up and held in detention since the start of the conflict eight years ago, which has left at least 20,000 dead in the country's remote northeast.
To date, just 13 people have been put on trial and only nine convicted for their links to the Islamist insurgency, according to official figures.
The most high-profile current case is that of Khalid Al-Barnawi, a leader of the Boko Haram offshoot Ansaru, who is charged with the abduction and murder of 10 foreign nationals.
Nigeria's justice ministry announced the start of the trials at the end of last month, saying four judges had been assigned and that defendants would have legal representation.
Some 1,670 detainees at a military base in Kainji, in the central state of Niger, will be tried first followed by 651 others held at the Giwa barracks in the capital of the northeastern state of Borno state, Maiduguri.
"It's the first significant trial of Boko Haram suspects," Matthew Page, a former US State Department analyst and a specialist on Nigeria, said.
But he told AFP that while "positive" it was still a "very small step", as many of the detainees had been held in custody for years, without access to a lawyer or ever having appeared before a judge.
How the long-awaited trials will be held also raises questions, particularly about transparency.
A justice ministry source said no media would be allowed on security grounds and that although civilian courts, they would be held in military facilities.
Umar Ado, a defence lawyer based in Nigeria's biggest northern city, Kano, said that was "as good as denying the public the right to know how the trial is carried out".
"It sends the wrong signal that justice is not served or the process is compromised," he added.