A Nigerian Shia group banned by the government said police killed 12 of members and wounded 10 others during marches in the north of country to mark the religious commemoration known as Ashoura.
Spokesman Ibrahim Musa said the Shia marchers were killed in the northern states of Kaduna, Bauchi, Gombe, Sokoto, and Katsina on Tuesday.
"The Islamic Movement in Nigeria has confirmed the killing of at least a dozen Ashoura mourners across the nation during the peaceful Ashoura mourning procession today," said Musa.
The group, the Islamic Movement of Nigeria (IMN), was banned in July after a series of deadly clashes with police. IMN said the police were responsible for the deaths of at least 20 people in July but the police gave no death toll.
Police in the northern city of Kaduna, where IMN said three were killed and 10 injured on Tuesday, disputed the account and said it dispersed marchers "professionally".
A national police spokesman did not immediately respond to a request for comment.
The group was marching to mark Ashoura, the day in Islamic tradition when the Prophet Mohammed's grandson, Imam Hussein, died in battle in 1680.
Police had warned IMN members not to march, saying any gathering or procession by group members is "ultimately illegal and will be treated as a gathering in the advancement of terrorism".
IMN said police attacked its marchers on Tuesday and, in Katsina, opened fire on them. It said members were killed in Bauchi, Gombe and Sokoto states, all in northern Nigeria, but marches in the capital, Abuja, and other northern states ended without incident.
Clashes with police in the last few weeks followed calls by the group for its leader to be released from police detention.
Their leader, Ibrahim el-Zakzaky, has been held since 2015 when government forces killed about 350 people after storming an IMN compound and a nearby mosque.
While roughly half of the nearly 200 million Nigerians are Muslim, mostly concentrated in the north of the country, Shia are a minority.
Last week the United Nations special rapporteur on extrajudicial, summary or arbitrary executions said she had not been presented with any evidence to suggest IMN was weaponised and posed a threat to Nigeria.