Amnesty International has accused Nigeria's military of deliberately shooting dead more than 350 Muslims and burying them in mass graves. Nigerian officials claimed the protesters wanted to kill the head of the army.
In a new report, the rights group said Nigeria's army had acted "unlawfully" by shooting "indiscriminately" at unarmed protesters from the Shiite-led Islamic Movement of Nigeria (IMN).
The alleged massacre took place last December during a visit by the pro-Iranian cleric Ibrahim Zakzaky to a religious ceremony in the northern city of Zaria.
The IMN has said the army's attack was preplanned after the protesters refused to allow the chief of army staff's convoy to pass.
The following day the army raided several buildings connected to the group.
In its report, Amnesty said, "It is not clear why the army launched such a 'military operation' in response to a law and order situation." It added that the Nigerian army provided "no evidence to substantiate its claim that IMN protesters attempted to assassinate the Chief of Army Staff."
The report accused the Nigerian military of burning people alive, razing buildings and dumping victims' bodies in mass graves.
Most of the evidence was "meticulously destroyed," Amnesty said, accusing soldiers of trying to cover up the carnage by limiting access to conflict sites.
Amnesty provided satellite images purporting to show the location of a "possible mass grave" in the Mando area near the state capital, Kaduna, approximately 80 kilometers (50 miles) away.
The rights group carried out research in February, conducting interviews with 92 people, including alleged victims and their relatives, eyewitnesses and medical staff.
The Nigerian military has been repeatedly accused of abuses against civilians in its fight against the Sunni Muslim jihadists Boko Haram. But military officials have maintained its troops acted appropriately.
Defense spokesman Brigadier General Rabe Abubakar said Amnesty's report was "unfair" as the military had not been consulted before its publication.
Nigerian President Muhammadu Buhari pledged to investigate evidence of war crimes, but Amnesty complained that "to date no concrete steps have been taken to end endemic impunity for such crimes."
Last week, a Nigerian public official said that 347 people, including women and children, were buried in a mass grave under military supervision and authorized with a court warrant.
The testimony came at one of several public inquiries set up to investigate the circumstances of the violence, which has led to scores of IMN supporters being charged.
Nigerian media on Thursday said prosecutors in Kaduna were seeking the death penalty for 50 IMN members for killing a soldier in the initial incident in Zaria.