Nafiu Mohammed and Suleiman Haruna said the explosion happened at the Farm Centre market shortly after 4:00 pm (1500 GMT) and rescuers were on the scene dealing with casualties.
“I was preparing for the afternoon prayers when there was an explosion deep inside the market at exactly 4:10 pm,” said Mohammed.
“Police officers from a nearby police station mobilised to the scene while traders in confusion locked up their shops and moved out of the market.
“We are now at a distance while the rescue vehicles are taking the victims to hospital. We have no idea of the number of casualties.”
Officers fired warning shots as a deterrent for people to stay back from the market, which is typically packed with traders and shoppers.
Haruna also said it was not clear how many people had been affected but he added: “I saw ambulances leaving the market.”
Nearly 1,500 people have been killed in northern and northeast Nigeria since President Muhammadu Buhari came to power on May 29, according to an AFP tally.
Buhari has given his military commanders until next month to bring Boko Haram’s six-year insurgency to an end but there has been waves of bombings, mainly against civilian “soft” targets.
On Tuesday evening, at least 32 people were killed and some 80 others injured in an explosion at a packed lorry park in the Adamawa state capital, Yola.
It was the first suspected Boko Haram attack in Nigeria since a twin suicide attack in Yola on October 23 and revived fears of a fresh round of carnage.
Kano has been relatively spared from the violence in the last six months, a fact attributed in part to its security network using local networks of traditional chiefs and the security agencies.
On July 6, a girl thought to be aged just 13 blew up outside a mosque in the city, killing only herself.
In November last year, at least 120 people were killed and 270 others wounded when two suicide bombers blew themselves up and gunmen opened fire during weekly prayers at Kano’s Grand Mosque.
The attack is thought to have been revenge for traditional leader the Emir of Kano’s earlier call for citizens to take up arms against the Islamist militants.