Gunmen set off three bombs and opened fire on worshippers at the main mosque in north Nigeria's biggest city Kano on Friday, Nov 28, killing at least 81 people, witnesses and officials said, in an attack that bore the hallmarks of Islamist Boko Haram militants, Reuters reported.

Blasts from the coordinated assault rang out as scores of people packed into the ancient building's courtyard for afternoon prayers.

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.

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.

President Goodluck Jonathan said in statement that he would "not to leave any stone unturned until all agents of terror undermining the right of every citizen to life and dignity are tracked down and brought to justice."

The old mosque and palace date back centuries to when Kano was one of several Islamic empires thriving off trade in gold, ivory and spices from caravan routes connecting Africa's interior with its Mediterranean coast -- glory days of Saharan Islam that Boko Haram says it wants to recreate.

The insurgency has forced more than one million people to flee 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.