Muhammadu Buhari said Wednesday no elections were conducted in the country's Niger Delta region and in the southeastern parts of the country and that his supporters weren't allowed to vote, Voice of America reported.
But Buhari urged Nigerians to remain calm and law-abiding, saying questions of voting irregularities were being pursued with the country's electoral commission. Nigerian President Goodluck Jonathan won re-election in the vote, which international monitors said was largely free and fair.
Riots broke out in the predominantly Muslim north after news that Jonathan, a Christian, defeated Buhari, a Muslim.
Witnesses reported seeing charred bodies along roadways, and burnt-out homes and churches, VOA reported. Media reports indicate up to 50 people may have died in post-election violence, although government officials and relief agencies haven't released casualty figures, Voice of America said.
None of Nigeria's opposition parties signed the final election results and Buhari's party filed a formal challenge.
Election returns indicate Jonathan received more than 22 million votes in Saturday's polls, nearly twice the number of Buhari, who collected about 12 million votes. Jonathan received 57 percent of the votes cast, enough to avoid a run-off.