Authorities have tightened security in northern Nigeria following a series of attacks blamed on Boko Haram group.

Armed men have staged an overnight raid on a prison in the town of Bauchi in Nigeria, freeing hundreds of prisoners and leaving four people dead.

Authorities on Wednesday said the men were members of an armed group known as Boko Haram, which seeks to institute sharia (Islamic law) in Nigeria.

The attackers, wielding AK-47 assault rifles, fired on the prison guards as they were breaking their daily Ramadan fast on Tuesday evening, Al Jazeera's Yvonne Ndege reported from Abuja, Nigeria's capital.

Between 800 and 900 of the prisoners broke out of the jail, including 200 inmates believed to be Boko Haram members, Ndege said. Four people were wounded in the clash.

Authorities have arrested 13 suspects in connection with the prison break, Ndege said, although residents in and around Bauchi remain afraid that Boko Haram will launch other attacks.

Security tightened

Danlami Yar'Adua, Bauchi's police commissioner, told the Reuters news agency that two of the dead were "bodyguards" and that the attackers had set part of the prison on fire.

One Bauchi resident told Reuters the attackers had numbered around 50. Police and military officers manned new checkpoints on the road out of Bauchi on Wednesday, hoping to catch escapees, the Associated Press reported.

Boko Haram - which means "Western education is sin" in the northern Hausa language - was blamed for riots and attacks in the north last summer that left more than 700 people dead.

Following the killing of several policemen by motorcycle-riding assailants over the past week, the government has tightened security in the north and northeast, and residents are fearful of a new wave of violence.

The Tuesday prison break coincided with the government's announcement that it would hold its presidential election on January 22.