At least 81 people have been killed and 150 others injured after two bombs exploded outside the central mosque of the Nigerian city of Kano.

Friday's attacks occurred outside the Emir's palace mosque just as residents were packing into the area for prayers, police said.

Witnesses said they heard gunshots but did not know who was shooting.

"Two bombs exploded, one after the other, in the premises of the Grand Mosque just seconds after the prayers had started," Aminu Abdullahi, a local resident, said, adding that a third went off nearby.

The AFP news agency put the death toll at 120.

Al Jazeera's Rawya Rageh, reporting from Yola in nearby Adamawa state, said the blasts struck the centre of Kano, northern Nigeria's biggest city.

"It was a very brazen attack in the centre of the city. The crowds at the mosque are usually the largest in the city during Friday prayers," she said.

Palace officials have said Emir Muhammad Sanusi II, one of the highest ranking Islamic figures in Nigeria, is currently out of the country.

The blasts came after a bomb attack was foiled against a mosque in the northeastern city of Maiduguri earlier on Friday, five days after two female suicide bombers killed over 45 people in the city.

Emmanuel Ojukwu, a national police spokesman, told AFP news agency that the bombers blew themselves up in quick succession, then "gunmen opened fire on those who were trying to escape".

Ojukwu said he did not know whether the suicide bombers were male or female, and did not give an exact figure on the number of attackers. 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.

As night fell, hundreds of people were desperately trying to use the lights on their mobile phones to identify loved ones.

Boko Haram suspected

There was also no immediate claim of responsibility, but suspicion is likely to fall on Boko Haram, the armed group which for five years has waged a campaign to carve out a separate state in the north ruled by Islamic law.

The Grand Mosque and palace date back centuries to when Kano was one of several Islamic empires thriving on 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.

Islamic leaders in Nigeria sometimes shy away from direct criticism of Boko Haram for fear of reprisals.

However, Sanusi, angered by atrocities such as the kidnapping of 200 schoolgirls from the village of Chibok in April, has become an increasingly vocal Boko Haram critic.

Last week the respected Muslim figure called on the people of Nigeria to defend themselves against Boko Haram.

"These people [Boko Haram], when they attack towns, they kill boys and enslave girls … People must stand resolute," Sanusi told the Daily Post website.

"People should be sensitised on the importance of being on the alert. And they should prepare, they should acquire what they will defend themselves with."

Ban Ki-moon, the UN secretary-general, condemned the attack, saying "there can be no justification for attacks on civilians".

The US decried what it called a "horrendous" attack, adding that it "stands with the Nigerian people in their struggle against violent extremism".

Jen Psaki, State Department spokesperson, said: "These attacks, while as yet unclaimed, have all the hallmarks of Boko Haram and the group's disregard for human life as it continues in its efforts to destabilise Nigeria."