Amnesty International on Friday accused the Nigerian army of having massacred more than 350 supporters of a Shiite Muslim minority group.
The men, women and children were killed in Zaria, 270 kilometres north of the capital, Abuja, in December, the human rights group said in a report.
The confrontation started when hundreds of supporters of the Islamic Movement in Nigeria (IMN), some of them wielding batons and knives, blocked a road in front of their headquarters and refused to let pass a convoy of the army chief of staff.
Soldiers opened fire, killing at least seven protesters, according to the report. A soldier was reportedly also killed.
The army has claimed that IMN supporters attacked the convoy, while IMN has said that its members only objected to the presence of soldiers in front of their headquarters.
Soldiers were later deployed to the city where they reportedly killed IMN supporters.
On April 11, authorities in Nigeria's Kaduna state admitted that they had secretly buried the bodies of 347 people in a mass grave two days after the confrontation, according to the report.
Nigerian army spokesman Sani Usman called the report “biased and unsubstantiated.”
He urged Amnesty not to “jump to a conclusion” before a judicial commission of inquiry and the National Human Rights Commission had investigated the incident.