ABUJA (Reuters) - At least 20 members of a Nigerian Shi'ite organisation have been killed this week, group spokesmen said, during a series of protests that show little sign of ending despite the increasing death toll and authorities saying they have boosted security.
The death toll may be as high as 25, one spokesman said.
Police did not respond to repeated attempts to reach them for comment.
Members of the Shi'ite Islamic Movement of Nigeria (IMN) have been marching in the capital Abuja calling for the release of their leader, Ibrahim Zakzaky, who has been in detention since 2015 despite a court order to release him.
Nigerian police met the protests with gunfire and tear gas. A journalist and a senior police officer also died after Monday's march.
Most of the people killed after security forces opened fire on the group on Monday, with four others having since died in police custody from their bullet wounds, the spokesmen said.
"More might die in police custody, because there are at least 15 people who are in the detention centre with various degrees of bullet wounds, without medication," one said.
Police said on Friday they had bolstered security across the country in the face of the ongoing protests, which they say are violent and unruly.
They "advised all would-be protesters to ensure they express their grievances within the ambit of the law," according to the statement.
Zakzaky has been held in detention since December 2015, when the army killed roughly 350 of his followers at his compound and a nearby mosque and burial ground in northern Kaduna state.
Zakzaky faces trial on charges of murder, culpable homicide, unlawful assembly, disruption of public peace and other offences following the 2015 violence. He has pleaded not guilty.
(Reporting by Garba Muhammed, Paul Carsten and Abraham Achirga; Additional reporting by Alexis Akwagyiram; Editing by Andrew Cawthorne)