Gunshots rang out in the affluent Ikoyi neighbourhood of Nigeria's commercial capital Lagos on Thursday, one witness said.
Smoke was also seen rising from the direction of Ikoyi correctional facility, a prison, the witness told Reuters.
A Reuters witness saw two trucks carrying police heading towards the prison.
The city of 20 million is under a round-the-clock curfew imposed after nearly two weeks of protests against police brutality - the West African nation's biggest wave of unrest since the end of military rule in 1999.
Several states are also under curfew and the oil-producing Delta state said it would enter a 48-hour curfew from 17:00 on Thursday.
Unrest escalated on Wednesday as groups of young men and armed police clashed in some neighbourhoods following a shooting on Tuesday night at a toll gate in the Lagos district of Lekki.
(Additional reporting by Angela Ukomadu in Lagos and Felix Onuah in Abuja; Editing by Kevin Liffey, Tomasz Janowski and Andrew Heavens)