Up to 20 explosions rock city of Kano in northern Nigeria.

Reuters - At least six people were killed in a string of bomb blasts on Friday in Nigeria's second city Kano and the authorities imposed a curfew across the city, which has been plagued by an insurgency led by the Islamist sect Boko Haram.

Smoke billowed from the police headquarters for the north in Kano after one blast blew out its windows, collapsed its roof and triggered a blaze that firefighters struggled to control.

A Reuters reporter counted three bodies at the scene and three more at the local passport office, which was surrounded by flaming debris.

Some residents ran around shouting and screaming following the attacks. There were at least four other explosions across the city in quick succession.

"I was on the roadside and I just heard a 'Boom!'. As I came back, I saw the building of the police zonal headquarters crashing down and I ran for my life," said local man Andrew Samuel.

There was no immediate claim of responsibility for the apparently coordinated attacks, which prompted the government to announce a dusk-to-dawn curfew.

Kano, like other northern cities in Nigeria, has been plagued by an insurgency led by Islamist sect Boko Haram, blamed for scores of bombings and shootings against mostly government targets that are growing in scale and sophistication.

Boko Haram became active around 2003 and is concentrated in the northern states of Yobe, Kano, Bauchi, Borno and Kaduna.

Boko Haram, which in the Hausa language of northern Nigeria means "Western education is sinful," is loosely modeled on the Taliban movement in Afghanistan.

The group considers all who do not follow its strict ideology as infidels, whether they be Christian or Muslim. It demands the adoption of sharia, Islamic law, in all of Nigeria.

Witnesses said the bomber of the police headquarters, which covers most of northern Nigeria, pulled up at the building on a motorbike then got off and ran at it holding a bag.

"We tried to stop him but he ran in forcefully with his bag. All of a sudden there was a blast. You can see for yourself the building is damaged," said a policeman at the scene.

Police said a second blast had hit Kano's passport office and another hit Zaria Road police station in the city.

"The ground was shaking with the explosion. We saw flames and smoke at the police station," said witness Umaru Ibrahim.

A source at the State Security Service said another bomber had tried to attack there but was gunned down before he could detonate his bomb.

Police and military roadblocks were erected in the city within minutes.

"We are trying to reach the scenes of these heavy blasts. Many of the roads are blocked now by security agents," said Abubaker Jibril, head of the National Emergency Management Agency (NEMA) for Kano, told Reuters.

A bomb attack on a Catholic church just outside the capital Abuja on Christmas Day, claimed by Boko Haram, killed 37 people and wounded 57.

The main suspect in that attack escaped from police custody within 24 hours of his arrest, and police have offered a 50 million naira ($309,600) reward for information leading to his recapture.

Police arrested Kabiru Sokoto on Tuesday and while they were taking him from police headquarters to his house in Abaji, just outside Abuja, to conduct a search there, their vehicle came under fire.

Last August a suicide bomber blew up the U.N. Nigeria headquarters in Abuja, killing at least 24 people.