Goodluck Jonathan, the Nigerian president, has been sworn in following elections last month that were seen as the country's fairest in two decades.
Heads of state from across Africa, foreign dignitaries, religious leaders and traditional rulers gathered on Sunday in Eagle Square in Abuja, the capital, for the ceremony and a military parade to mark the start of his four-year term.
Just hours after the swearing-in ceremony, officials said a bomb blast rocked a popular drinking spot by an army barracks in Bauchi, in Nigeria's north, killing a dozen people.
A rescue worker who asked not to be identified told the Reuters news agency his colleagues had counted 12 dead bodies and that around 25 people had been wounded by Sunday's blast.
Jonathan faces a divided nation after deadly post-election riots killed more than 800 people.
The rioting and massacres spread across the north of Africa's most populous nation and largest oil producer, with victims hacked, burnt and shot to death.
Mosques and churches were burned and people beaten after being pulled from cars.
Sunday's explosion in Bauchi hit the Mamy market on the city's edge, at around 8pm local time (19:00 GMT), Muhammed Indabawa, the police commissioner, said.
He said it was not clear who was responsible and that no arrests had yet been made.
"It was a very strong and powerful explosion," Yushua Shuaib, the spokesman for the National Emergency Management Agency (NEMA), told Reuters. He said the wounded had been taken to hospital but declined to comment on the death toll.
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A second, smaller explosion hit a beer parlour in Zuba on the outskirts of Abuja, although the cause was unknown and there were only three minor injuries, Shuaib said.
Bauchi neighbours Plateau state in Nigeria's "Middle Belt" where the mostly Muslim north meets the predominantly Christian south, a region beset by years of sectarian violence.
Hundreds of people were killed in northern towns last month in riots and reprisal killings after Jonathan, a Christian from the south, was declared winner of a presidential election, beating Muhammadu Buhari, a northern Muslim and former army ruler.
There were several bomb blasts at campaign rallies in the run-up to the April elections, most of them using home-made improvised devices and carried out by unknown assailants.
Jonathan, who is the first president from the oil-producing Niger Delta region, will be seeking to put the violence behind him.
However, in the north, suspicion remains as many accuse Jonathan's party of election-rigging and reject reports from observers calling the election a step forward for the country.
Abubakar Siddique Mohammed, who runs a think-tank in the northern city of Zaria where the home of the vice-president was torched in the riots, said: "Jonathan does not have legitimacy".
He said that he personally witnessed numerous cases of ballot fraud, and called election observers' claims of fairness lies.
Buhari alleged fraud in the election, but disassociated himself from the violence and did not make strong public statements against the riots as they unfolded.
Some of the distrust of Jonathan stems from the overturning of an internal ruling party arrangement that saw it rotate its candidate between the north and south every two terms.
Jonathan is serving what would have been the second term of late northern president Umaru Yar'Adua, and there is an expectation among the northern elite that at the next elections in four years, a northern candidate will take the ruling party ticket.
But should Jonathan succeed with his reform plans, particularly in privatising the power sector and ending chronic power shortages, popular opinion could again swing in his favour.