About 10 gunmen went on a shooting spree after the blast at the mosque, which is close to the Emir of Kano’s palace.
A rescue official, who spoke with Agence France-Presse, put casualty figures at almost 400, adding that 120 were killed and 270 hurt in the three explosions that rocked the mosque.
The emir was said to have travelled to Saudi Arabia late on Thursday night from Paris.
Witnesses said the incident happened just as residents were getting set for Friday prayers.
“The attackers have bombed the mosque. I saw people screaming,” a local reporter who resides in Kano, Chijjani Usman, told Reuters.
Another eyewitness told the British Broadcasting Corporation that he had counted over 50 bodies immediately after the blasts.
Also, the Editor, BBC Hausa Service, Mansur Liman, was quoted as saying that people had witnessed “horrible” scenes in a nearby hospital.
One eyewitness, who spoke with the BBC’s Focus on Africa, said, “The imam was about to start prayer when he saw somebody in a car trying to force himself into the mosque. But when people stopped him, he detonated the explosions. People started running helter-skelter.”
Another worshipper, Aminu Abdullahi, who also spoke to AFP, said, “Two bombs exploded, one after the other, in the premises of the Grand Mosque seconds after the prayers had started.”
“A third one went off in a nearby road close to the Qadiriyya Sufi order. The blasts were followed by gunshots by the police to scare off potential attacks.”
His account was backed up by another witness, Hajara Tukur, who said she lives nearby.
A senior rescue official confirmed that several bodies had been brought to just one Kano area hospital, while over 126 people had been admitted with injuries at three facilities.
“Those figures are going to climb,” he told AFP on condition of anonymity, as he was not authorised to speak to the media.
A military official, Capt. Ikechukwu Eze, also said scores of people were feared killed and many others injured in the attack.
A check by one of our correspondents at the mortuaries of both the Murtala Mohammed Specialist Hospital and the Nasarawa Hospital in the heart of the city showed they were filled with bodies of victims of the blasts while some of them littered the ground.
Meanwhile, two suspected bomber were said to have been arrested at the scene of the incident, just as youths took to the street to protest what they called negligence by the police for not doing enough to protect the lives and property of the people.
The Kano State Police Command’s Public Relations Officer, Mustapha Abubakar,confirmed the incident but declined to give the casualty.
The emir had recently called for people to arm themselves against Islamist militant group Boko Haram.
Earlier this month, the emir told a prayer meeting that people should “acquire what they need” to protect themselves.
The Boko Haram terrorist group has killed more than 2,000 people this year, according to rights groups’ reports.
The Kano blasts came after a bomb attack was also foiled at a mosque in Maiduguri on Friday morning; five days after two female suicide bombers killed over 45 people in the city.
The Emir of Kano last week doubted the Nigerian troops’ ability to protect civilians and end the insurgency.
This had prompted his advice to Kano residents to arm themselves against Boko Haram attacks.
The Emir of Kano is an influential figure in Nigeria, which is home to more than 80 million Muslims, most of who live in the North.
Boko Haram has repeatedly attacked Kano before. On November 14, a suicide bomb attack at a filling station killed six people, including three policemen.