More than 200 inmates were freed in a raid on a prison in Nigeria's troubled southeastern city of Onitsha, officials and residents said.

Official Nigerian radio reported that suspected "hoodlums" invaded the Onitsha prison after breaking into it and setting parts of the building on fire.

It was the second time this year that the prison had been attacked and its inmates released. More than 700 Onitsha prison inmates were liberated last February during unrest in the city.

A senior official at the prison, who simply gave her name as Ms Okpoko, told reporters that a total of 204 prison inmates were set free by the "hoodlums" who invaded the building at around 2:00 am (0100 GMT) on Monday.

One of four prison inmates who later returned, Bright Chukwujekwu, told journalists that the attackers, armed with guns, machetes, hammers, iron rods and acid, forced the gate of the prison to open.

They attacked and injured a number of prison officials who were on duty during the assault, said Chukwujekwu, who alleged that the invaders presented themselves as members of the recently banned Movement for the Actualisation of the Sovereign State of Biafra (MASSOB), a separatist group.

MASSOB spokesman David Chinedu denied to AFP any involvement of his group in the attack on the prison.

The attack on Onitsha prison came less than 24 hours after troops were deployed and a curfew imposed on the troubled city.

Onitsha, a major commercial centre in Anambra State, and the southeastern region, has been the scene of violent unrest since Friday.

Clashes between MASSOB members and police were reported to have left several people dead at the weekend.

The troop deployment and the week-long curfew were measures taken on Sunday to restore peace to the area.

The deployment, at the request of state governor Peter Obi, was for "security reasons, the need to protect lives and property," army spokesman Yusuf Mohammed told AFP earlier Monday.

According to press reports, at least seven people died in Onitsha's Upper Iweka district in clashes Friday between police and suspected MSSOB supporters.

But Anambra police spokesman Fidelis Agbo denied to AFP by telephone that anybody was killed in the unrest.

Obi had on Saturday banned MASSOB and two transport unions, widely believed to be used by opposition politicians to fan the crisis in Onitsha.

He said that the move was taken, "in order to ensure that Onitsha and the whole of Anambra State continue to enjoy peace."

The governor said then that he had formally requested President Olusegun Obasanjo to intervene in the crisis, which he said was a potential threat to national security.

MASSOB is an ethnic Igbo group campaigning for the resuscitation of "Biafra", which led a secessionist rebellion against the federal government between 1967 and 1970.

The war ended in January 1970 with the surrender of the Biafra warlords.

MASSOB says it is also fighting against an alleged marginalisation of Igbos since the war. Its founder and leader, Ralph Uwazuruike, and some of his followers face treason charges in an Abuja court.