Authorities accuse the Islamic Movement of trying to assassinate the chief of army staff, Lt. Gen. Tukur Buratai, a charge that the movement has denied.
The group was conducting an annual ritual to usher in the month of Maulud, the birth month of the Prophet Mohammad.
The army Sunday raided several buildings connected to the movement and the home of its leader, Ibrahim Zakzaky.
They arrested him and killed key members of the group, including Zakzaky’s second-in-command and his spokesman.
“As of yesterday, we had 60 corpses in our morgue,” Khalid Lawal, the chief medical director of Ahmadu Bello University Teaching Hospital, told Reuters by phone.
Residents said they heard loud blasts during the raid.
Meantime, Iran condemned the attack Monday and summoned Nigeria’s representative there, according to its state news agency. Foreign Minister Mohammad Javad Zarif called on the Nigerian government to take “prompt and serious” action.
The Shiite sect said that hundreds of their members were killed. The army took most of the bodies away, making it impossible to verify the claim.
Spokesmen for Nigeria’s presidency declined to comment and referred Reuters to the army spokesman, who did not answer multiple requests for comment.
A similar altercation between the sect and the army occurred last year during a procession. Zakzaky said that 30 followers and three of his children were killed.