A female suicide bomber has blown up herself in northern Nigeria's Gombe city, minutes after President Goodluck Jonathan left a campaign rally there.
At least one person was killed and 18 others were wounded in the blast, police and hospital sources said.
President Jonathan is standing for re-election on 14 February against former military ruler Muhammadu Buhari.
Militant Islamist group Boko Haram has stepped up its attacks in the run-up to the contest. It has not commented on the blast.
Explosions have also ripped through court buildings in three towns in oil-rich southern Nigeria in what police described as co-ordinated attacks.
Dynamite was suspected to have been used in the attacks in Port Harcourt city and the towns of Isiokpo and Degema in oil-rich Rivers State, regional police spokesman Ahmad Muhammed said.
There were no casualties, but the court building in Degema was "razed down and documents burnt", he is quoted by Nigeria's privately owned Daily Trust newspaper as saying.
In the blast in Gombe, the bomber blew herself up near a car, Gombe state police spokesman Fwaje Atajiri told the BBC.
He said a female passerby had been killed, contradicting earlier reports that three people had been killed in the blast.
Jonathan addressed a rally in the north-eastern city a day after it was hit by two blasts that killed at least five people.
A local journalist told AFP the latest blast had led to unrest in Gombe, with angry youths attacking supporters of Jonathan's People's Democratic Party (PDP).
"They were shouting and denouncing the president's visit which they blamed for the attack," he added.
A report in the Nigerian paper The Vanguard says the president and Buhari have cancelled scheduled election rallies in Damaturu in Yobe state and Maiduguri in Borno state respectively.