Image copyright AFP/Boko Haram

The first in a series of trials of more than 6,600 people, accused of being members of militant Islamist group Boko Haram, has opened in Nigeria.

The trials are being held in secret by civilian court judges at a military facility in north-central Kainji town.

Rights activists say they are concerned about the lack of transparency in what has been described as the biggest terrorism trials in Nigeria’s history.

Some 20,000 people have been killed in Boko Haram’s eight-year insurgency.

Only nine people have been convicted so far of being involved in the rebellion.

Four judges have started the trials at the military centre in Kainji, sources at the ministry of justice told the BBC’s Ishaq Khalid in the capital, Abuja.

Up to 1,670 people will be tried in the coming weeks with a further 5,000 people after that, our reporter says.

More than 1,600 suspects are being held at the centre, where many have been for years.

In a report in 2015, Amnesty International said that military forces had arbitrarily detained about 20,000 people as part of its campaign to end the insurgency.

The trials are likely to last for months, or even years, because of the huge number of suspects who will be tried individually, Justice Minister Abubakar Malami said.