Abubakar has rejected the result, calling the election a “sham” and vowed to file a legal challenge.
Nigeria, Africa’s most populous nation, has a history of election violence and malpractice, including ballot-snatching, voter intimidation and vote-buying.
During the ballot, which Buhari won by nearly four million votes, some 53 people were killed and dozens of others were injured, according to monitoring groups.
In 2011, more than 1 000 were killed in post-election violence in northern Nigeria.
AFP