Riots have broken out in the two big northern Nigerian cities of Kano and Kaduna as presidential poll results show Goodluck Jonathan is set to win.
With nearly all the votes counted, the incumbent – a Christian from the oil-producing Niger Delta – has almost twice the number of his main rival.
Young supporters of Muhammadu Buhari, who is popular in the north, have been clashing with police.
The African Union observer team said it was Nigeria’s best poll for decades.
Mr Jonathan was appointed to the presidency last year, upon the death of incumbent Umaru Yar’Adua, whom he had served as vice-president.
He staked his reputation on the election, repeatedly promising it would be free and fair.
Results so far put Mr Jonathan on track to become the country’s first elected president from the Niger Delta.
The BBC’s Mansur Liman in Kano says smoke is billowing over the skyline as angry youths burn tyres across the city, the largest in the north.
They feel that the elections have been rigged in some areas of the south where there is a discrepancy between turnout and results, he says.
Police, who have appealed for calm on state radio, have fired tear gas in some areas.
School children have been sent home and businesses are closing.
There are also reports that the houses of prominent politicians of Mr Jonathan’s PDP party have been burned, our correspondent says.
In Kaduna, police are using tear gas and live ammunition to quell similar protests, the BBC’s Abdullahi Kaura Abubakar reports.
To win at the first round, a candidate needs at least 25% of the vote in two-thirds of Nigeria’s 36 states.
According to regional results, Mr Jonathan has passed that threshold in at least 24 states.
He has polled more than 22m votes, compared with 12m or so for former military leader General Buhari.
In Akwa Ibom state, Mr Jonathan was credited with winning 95% and in Anambra it was 99%. In his home state, Bayelsa, he took 99.63%.
“Figures of 95% and above for one party suggest that these are fabricated figures and, personally, they worry me because they pose serious questions on the credibility of the election,” Jibrin Ibrahim of the Centre for Democracy and Development told AFP news agency.
A spokesman for General Buhari, Yinka Odumakin, also said irregularities had taken place, but any challenge would come after the vote count.
Mr Jonathan’s campaign team said they would not comment publicly until the election commission had formally declared all the results in the capital Abuja. That announcement is expected later on Monday.
While past polls have been marred by widespread violence and vote-fixing, Saturday’s seemed to go generally smoothly.
Voters in many areas queued patiently for hours despite intense heat to cast their votes.
The head of the African Union observer team, former Ghanaian President John Kufuor, told the BBC he was satisfied.
“Nigeria hasn’t been served too well for decades electorally, but to our pleasant surprise we found the people of Nigeria generally are the security against this,” said Mr Kufuor.
“All of them co-operating to give the nation a befitting election.”
There has been tension and violence in other parts of the north and officials’ homes were burned, amid rigging allegations.
A curfew was imposed on Sunday in Gombe state because of rioting. Unrest was also reported in parts of Adamawa, Sokoto, Katsina and Bauchi states.
There were several explosions on polling day, including one at a hotel in Kaduna state and another in the Borno state capital of Maiduguri.