The Islamic Movement of Nigeria claims there is a plot to kill its leader, Ibrahim El-Zakzaky who has been in government custody since December 2015.
His detention followed clashes between his followers and troops of the Nigerian Army in Zaria Kaduna state, leading to the death of more than 300 IMN members.
El-Zakzaky is in detention alongside his wife, Zeenat.
The couple have been in detention in spite of a court order ordering their release.
The government said they are kept in protective custody for their own safety