The group of attackers used explosives to blast the administrative block of the facility and enter the prison yard in the southeastern city of Owerri, said police.
More than 1,800 inmates have escaped from a Nigerian prison after it was attacked by gunmen, officials have said.
Authorities are understood to think that the banned separatist group, the Indigenous People of Biafra (IPOB), was behind the attack, but a spokesman for the group has denied any involvement.
A total of 18,444 prisoners have escaped but six people are believed to have returned and 35 did not leave the prison while it was being assaulted.
The attackers arrived at the facility in the early hours as well as carrying explosives were also heavily armed with rifles, machine guns and rocket-propelled grenades.
The secessionist movement in the southeast is one of several serious security challenges facing President Muhammadu Buhari, including a decade-long Islamist insurgency in the northeast, a spate of school kidnappings in the northwest and piracy in the Gulf of Guinea.
Tensions have increased in the southeast in recent months in the wake of accusations that a paramilitary wing of IPOB, known as the Eastern Security Network, have been involved in clashes with the military.
“Preliminary investigations have revealed that the attackers… are members of the proscribed Indigenous People of Biafra (IPOB),” said Frank Mba, a spokesperson for the Nigeria Police Force.
But a spokesman for IPOB told Reuters in a phone call that the group was not involved. He said a statement would be issued in due course.
A Nigerian Correctional Service spokesperson said: “The Owerri Custodial Centre in Imo state has been attacked by unknown gunmen and forcefully released a total of 1,844 inmates in custody.”
Several police stations have been attacked in southeastern Nigeria since January, with large amounts of ammunition stolen. No groups have claimed responsibility for the attacks.
