The attack at the Grand Mosque in Kano, the biggest city in the mainly Muslim north of the country, came just as Friday prayers had started.
The mosque is attached to the palace of the Emir of Kano Muhammad Sanusi II, Nigeria's second most senior Muslim cleric, who last week urged civilians to take up arms against Boko Haram.
The blasts came after a bomb attack was foiled against a mosque in the northeastern city of Maiduguri earlier today, five days after two female suicide bombers killed over 45 people in the city.
Ojukwu said he did not know whether the suicide bombers were male or female, after a spate of attacks by women in recent months, and did not give an exact figure on the number of gunmen.
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But he said an angry mob killed four of the shooters in the chaotic aftermath. Witnesses in the city said they were set on fire.
An AFP reporter at the Murtala Mohammed Specialist Hospital morgue counted 92 bodies, most of them men and boys with blast injuries and severe burns.
But a senior rescue official said later that there were at least 120 dead and 270 wounded. Emergency workers were still trying to visit all hospitals, he added.
The Emir of Kano last week told worshippers at the same mosque that northerners should take up arms against Boko Haram, which has been fighting for a hardline Islamic state since 2009.
He also cast doubt on Nigerian troops' ability to protect civilians and end the insurgency, in rare public comments by a cleric on political and military affairs.