Abuja - The leader of Nigeria's minority Shi'ite Muslims accused the military on Saturday of killing three of his sons and 30 of his supporters when they opened fire on a protest the previous day.
The military said they were fired upon first by someone in the crowd and that they killed nine people.
The Shi'ites were protesting Israeli attacks on Gaza and also marching to observe of a religious holiday, in the northern city of Zaria, where Ibrahim Zakzaky's Islamic Movement of Nigeria is based, when a troop convoy tried to pass.
"Soldiers opened fire on our people at Kuban bridge killing 30 of our people, among which were three of my sons," Zakzaky said in a statement. He accused the government of ordering the attack and said two of his supporters were executed after being arrested.
"I however appeal to our people to remain calm," he said.
Defence spokesman Major-General Chris Olukolade said: "the convoy of soldiers who were held up in the traffic created by the procession were fired at from within the procession."
They returned fire, killing nine of the marchers and two soldiers were wounded, he said by telephone.
Though most of Nigeria's tens of millions of Muslims are Sunni, there are several thousand Shi'ites, mostly followers of Zakzaky, who established a movement inspired by the 1979 Iranian revolution.
The Islamic Movement has some members who are armed, although it seen as generally peaceful.
However, it does not recognise Nigeria's traditional Islamic authorities or government as legitimate, saying it takes orders only from God.
And like the Sunni Islamist movement Boko Haram, it attracts poor youths in largely Muslim north who are disenchanted with the corrupt political establishment.
The killing in police captivity of Boko Haram founder Mohammed Yusuf in 2009 during a crackdown in which some 800 other people also died transformed the sect from a clerical movement into a fully fledged armed insurrection.
The rebellion has since killed many thousands and Boko Haram is still holding more than 200 schoolgirls kidnapped in April.
The police on Saturday said it had "ordered effective and extensive deployment of officers and men of the Nigeria Police Force and other special operatives" to protect against possible militant attacks during the Sallah Muslim holiday on Monday and Tuesday.