Rivers state police spokesman Nnamdi Omoni said 32 suspects were in custody after two days of what he said were “violent protests” in the state capital, Port Harcourt.
“They will be taken to court after investigations,” he told AFP by telephone.
Members of the Indigenous People of Biafra (IPOB) group have in recent days intensified their calls for a separate state for the dominant Igbo ethnic group in southeast Nigeria.
That has sparked clashes with the military and police.
Omoni said a police sergeant attached to the force riot squad was killed in Port Harcourt on Wednesday when IPOB members seized his rifle.
Several other officers were injured and a police patrol van was burnt.
Nine of the 32 were arrested in connection with the incident, while the 23 others were held on Tuesday after an attack on the city’s police training school, he added.
Tensions are running high between IPOB supporters and the military, which this week began operations against rising crime across the south.
In Abia state, next to Rivers, a police station in the commercial hub of Aba was burnt down between 6 a.m. and 7 a.m. on Thursday, said police spokesman Geoffrey Ogbonna.
“We are investigating the incident but no arrest has been made,” he added.
But he denied reports that the commissioner of police’s residence in the state capital Umuahia was attacked.
A three-day dusk-to-dawn curfew was imposed in Abia state on Tuesday to prevent clashes after IPOB said troops killed five of its members.
The army has denied the claims.
A video clip has been circulating online purportedly showing soldiers punishing IPOB members by forcing them to drink muddy water while stripped to the waist.
The army said it would look into the claims, vowing that any soldier found guilty of breaching its code of conduct “will face (the) full wrath of the military justice system.”
It also urged close public scrutiny of IPOB claims, warning the group will use “bogus, manipulated and photo-shopped photographs as well as video clips” to back their cause.
IPOB leader Nnamdi Kanu has been charged with treasonable felony and is currently on bail pending the resumption of his trial in the capital, Abuja, next month.
A previous unilateral declaration of an independent republic of Biafra in 1967 led to a brutal 30-month civil war that left more than one million dead, most of them Igbos.