Bomb attacks targeting security forces and gun battles killed at least 162 people in Nigeria's second-largest city of Kano, sources said, as bodies littered the streets on Saturday.

A curfew was imposed on Kano in Nigeria's mainly Muslim north after it exploded into violence on Friday evening, with eight police and immigration offices or residences targeted.

The main newspaper in the north said that a purported spokesman for Islamist group Boko Haram had claimed responsibility for the violence, saying it was in response to authorities' refusal to release its members from custody.

Scores of such attacks in Nigeria's north have been blamed on Boko Haram, though Friday's would be among the group's most audacious and well-coordinated assaults.

Some 20 huge blasts could be heard in the city as a suicide bomber struck a regional police office and a car bomb rocked state police headquarters after the attacker fled and was shot dead, police sources said.

A number of other police posts were targeted, including a secret police building, as well as immigration offices.

Gunfire shook a number of areas, and a local television journalist was among those shot dead as he covered the unrest.

"We have been receiving dead bodies since last night from relief agencies involved in the evacuation of bodies," an official at the city's main morgue said on condition of anonymity because he was not authorised to give out figures.

"At this moment we have 162 bodies in the morgue, and this figure may change because bodies are still being brought," he added.

A source with the Red Cross said his agency alone had counted 121 dead.

A journalist counted at least 80 bodies in the main morgue, many of them with gunshot wounds, and said there were piles of other corpses he was unable to count.

Around 100 people waited outside the morgue to collect their relatives' remains.

Residents also reported bodies in the streets, as officials from the Red Cross and the National Emergency Management Agency worked to pick up the corpses.

"Between my house and the police headquarters along this street, I have counted 16 dead bodies that litter the streets, six of them policemen," Naziru Muhammad, who lives near state police headquarters, said Saturday morning by phone.

A police source on condition of anonymity said dozens were killed.

"There are heavy casualties around the police headquarters," the source said. "A lot of civilians have been shot by the attackers. It's difficult to give a death toll, but the number of the dead runs into dozens."

Details began to emerge of the attacks, which were said to involve at least two suicide bombers.

At state police headquarters, a would-be suicide bomber sought to join a police commissioner's convoy, the police source said, but jumped out of the car and tried to escape when officers opened fire.

The source said he was shot dead, but according to a resident, the car rolled over and a huge explosion followed.

The attacks in Kano, which had escaped the worst of the violence blamed on Boko Haram in recent months, sent residents fleeing in fear of what would come next.

President Goodluck Jonathan declared a state of emergency on December 31 in parts of four states hard hit by attacks attributed to Boko Haram, but Kano was not included.

Most of the recent major attacks have taken place in the country's northeast.