The armed men used guns and dynamite and also attacked a police facility in the area.
Armed men in the early hours of Monday attacked a correctional facility in Owerri, the Imo State capital, freeing some inmates.
They also proceeded to the Imo State Police Headquarters where they set ablaze the building and destroyed all vehicles within the vicinity.
Both the Imo State Police spokesperson and the state Commissioner for information were not available for comments as they did not respond to calls and SMS by PREMIUM TIMES.
PRNigeria, a news agency with close ties to security agencies, reported that the armed men used guns and dynamite to carry out the attack.
The news agency said the overnight attack was carried out by the members of the secessionist group, the Indigenous People of Biafra (IPOB).
It said the group, which had been alleged to be behind the killing of security personnel in the South-east, stormed the facilities and set alight some offices, structures and vehicles within the vicinity.
"Heavy gunshots and explosions could be heard in the area when they arrived with sophisticated weapons including dynamite and guns.
"Meanwhile while some of the inmates in the Correctional facility refused to go others have voluntarily returned," a security source told the news agency.
PRNIgeria also quoted another report as saying, "At about 0100hrs of date, armed men suspected to be members of IPOB/ESN criminal gang in their numbers invaded Owerri Prisons mine using dynamites and other heavy explosives, gained entrance and released some inmates in the prison.
"The hoodlums also attacked the Police State Headquarters Owerri while firing consistently with AK 47 rifles and explosives, released some suspects detained in the SCID cell and put fire on so many vehicles parked at the Headquarter premises including the SCID office building.
"The onslaught/ operation lasted close to three(3) hours."
It said more than 30 inmates have already returned to the correctional facility.
The News Agency of Nigeria (NAN) had earlier reported the police spokesperson, Orlando Ikeokwu, as saying that the security agencies were on top of the matter.
Mr Ikeokwu said only the State Criminal Investigation Department of the command headquarters was attacked.
He also confirmed that some vehicles were set ablaze by the gunmen.
NAN also reported that security agents had taken charge of the major streets in the state capital.
Police as target
PREMIUM TIMES had reported that the police blamed recent attacks on police facilities and personnel in the South-south and South-east on IPOB members with some suspects already arrested.
Several police officers have been killed lately in a string of attacks by gunmen, mostly around the two regions.
Police stations are often the target of the attacks, which appeared coordinated. The gunmen would raid the armoury, cart away police rifles, and then set the stations on fire, during the attacks. Police vehicles on patrol have also been targeted by gunmen.
The police recently announced the arrest of the suspects who were said to be behind the attacks.
The police spokesman, Frank Mba, a commissioner of police, said the arrest was "an outcome of painstaking, deliberate and tireless" collaborative effort between the police, the Nigerian Army, and the Nigerian Air Force.
The police named the suspects to include Ugochukwu Samuel, otherwise known as Biggy, 28, from Arochukwu Local Government Area, Abia State; Raphael Idang, 31, from Odukpani Local Government Area, Cross River State; Cletus Egole, otherwise known as Alewa, 60, from Orlu in Imo State; and Michael Uba, 33, also from Imo State.