Gunmen set off three bombs and opened fire on worshippers at the main mosque in north Nigeria's biggest city on Friday, killing at least 92 people, witnesses and officials said, in an attack that bore the hallmarks of Islamist Boko Haram militants.

Blasts from the co-ordinated assault rang out in Kano as scores of people packed into the ancient mosque building's courtyard for afternoon prayers. "These people have bombed the mosque. I am face to face with people screaming," local reporter Chijjani Usman said.

The mosque is next to the palace of the emir of Kano, the second-highest Islamic authority in Africa's most populous country and a vocal critic of Boko Haram. The emir, former central bank governor Lamido Sanusi, was not present.

Nigeria Mosque bombing

Bomb squad experts and security personnel inspect wreckage at the scene of Friday's multiple bombings in Kano, northern Nigeria. (Reuters)

Journalist Anna Cunningham, who spoke to CBC News earlier Friday, reported that the death toll has reached at least 92, based on the number of bodies counted at just one hospital.

Officials say at least another 100 are injured.

"Three bombs were planted in the courtyard to the mosque and they went off simultaneously," a security source who declined to be named said.

A staff member at the palace who also witnessed the attack said: "After multiple explosions, they also opened fire. I cannot tell you the casualties because we all ran away."

After the attacks, angry youths blocked the mosque's gates to police, who had to force their way in with tear gas.

Boko Haram, a Sunni jihadist movement which is fighting to revive a medieval Islamic caliphate in northern Nigeria, regards the traditional Islamic religious authorities in Nigeria with disdain.

It has attacked mosques that do not follow its radical ideology in a bloody near six-year campaign that has also targeted churches, schools, police stations, military bases and government buildings, killing thousands.

In a statement Friday evening, a spokesman for the UN secretary general said Ban Ki-moon condemned the attacks and has called on Nigerian authorities to "swiftly bring the perpetrators to justice."

"The secretary general reaffirms the full support of the United Nations to Nigeria’s efforts to fight terrorism and to provide security to the population," the statement said. 

A million people displaced

The insurgency has displaced over one million people during its campaign focused on Nigeria's northeast, the Red Cross told reporters on Friday, an increase on a September UN refugee agency estimate of 700,000.

Islamic leaders sometimes shy away from direct criticism of Boko Haram for fear of reprisals. But Sanusi, angered by atrocities such as the kidnapping of 200 schoolgirls from the village of Chibok in April, has been increasingly vocal.

He was quoted in the local press as calling on Nigerians this month to defend themselves against Boko Haram. During a broadcast recitation of the Koran he was reported to have said:

"These people, when they attack towns, they kill boys and enslave girls. People must stand resolute.... They should acquire what they can to defend themselves. People must not wait for soldiers to protect them."

Persisting insecurity is dogging President Goodluck Jonathan's campaign for re-election to a second term in February 2015. He has asked parliament for approval to extend an 18-month-old state of emergency in the northeast.

With files from CBC News