The head of state had been addressing supporters of his ruling Peoples Democratic Party (PDP) in Gombe city and had just left the venue when the blast happened in a car park outside.
Rescue workers and health officials said the bodies of two women were brought to the Gombe State Specialist Hospital with 18 people who were injured. The target of the attack was not clear.
"We have evacuated two bodies of females we believe were suicide bombers behind the blast," said the rescue official, who asked to remain anonymous because he was not authorised to speak to media.
Nigeria's government meanwhile claimed that it had retaken Gamboru and four other towns after a joint weekend offensive by its military, civilian vigilantes and forces from Chad and Cameroon.
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"Our troops are in control after operations which had the active support of volunteers (vigilantes) and our friendly neighbours," national security spokesman Mike Omeri told AFP.
The military progress comes after Chadian fighter jets have for three days been bombing Boko Haram positions in the town of Gamboru, according to an AFP reporter in Fotokol, a Cameroonian town less than one kilometre (mile) from Gamboru. The situation appeared quiet by today evening, he said.
The election is expected to be the closest since Nigeria returned to civilian rule in 1999, with the prospect of the PDP being dumped out of power for the first time in 16 years.
Boko Haram is in control of most of Borno state and has effectively surrounded Maiduguri, which is seen as one of the few places left in the state where voting could feasibly still take place.