The police commissioner for the state Bala Hassan said a suspected bomber and two civilians also died in the attack but eyewitnesses said the toll could be much higher.
A resident of the city claimed to have seen five police vehicles driven away with bodies of those killed in the bombing which is the second within the metropolis on the same day.
The first, which happened much earlier, exploded prematurely killing the bomber only.
The suicide attack bears the signature of an Islamic sect, Boko Haram which has been been waging a bloody war against Nigeria's government.
The sect wants to turn the country to an Islamic state with Sharia rule.
Its suicide bombing and shooting campaign this year alone has led to the deaths of hundreds of people.