Supporters of an imprisoned Shiite cleric clashed Tuesday with security forces around Nigeria's parliament building in the latest violent confrontation between the group and the authorities.
The Islamic Movement of Nigeria (IMN) said two protesters were killed while the police said two officers were shot in the legs as demonstrators tried to "force their way into the National Assembly".
Pro-Iranian cleric Ibrahim Zakzaky has been in custody since fighting between his supporters and the army broke out in December 2015.
Zakzaky's supporters have repeatedly protested in Abuja and several northern cities against the continued detention of their leader.
Witnesses told AFP that protesters seized a rifle from a police officer manning the main entrance to the assembly, located in the country's institutional capital Abuja.
"They seized the rifle from one of them and shot another policeman standing close by," one witness said.
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Security forces responded by firing at the protesters, hitting some of them, witnesses said.
An AFP journalist saw traces of blood on the road leading to the assembly and cars with their windscreens smashed by fleeing demonstrators.
Police said two officers received gunshot wounds and six others were hit by clubs and stones.
IMN spokesman Abdullahi Musa Mohammed told AFP the security forces fired teargas before shooting at the protests with live ammunition.
"They killed two and so many were injured," he said.
The police said 40 IMN members were arrested and that they had ordered "24-hour police surveillance" across the capital.
The National Assembly hurriedly suspended its plenary session due to the violence.
In October, the IMN and human rights groups said more than 40 people were killed when the security forces opened fire on crowds on the outskirts of the capital.
The army maintained six people died and that soldiers acted in self-defence.
Zakzaky has been at loggerheads with Nigeria's secular authorities for years because of his call for an Iranian-style Islamic revolution. Northern Nigeria is majority Sunni Muslim.