Riots have broken out across northern Nigeria as presidential poll results show Goodluck Jonathan is set to win.
Homes of supporters of Mr Jonathan, the incumbent, were attacked in the cities of Kano and Kaduna.
Young supporters of Muhammadu Buhari, who is popular in the north, have been clashing with police and military.
They feel that the elections have been rigged in some areas of the south where there is a discrepancy between turnout and results.
With nearly all the votes counted, Mr Jonathan - a Christian from the oil-producing Niger Delta - has almost twice the number of his main rival.
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.
In Kano, the largest city in in the north, homes displaying posters of Mr Jonathan were set on fire, and gangs of young men roamed the streets shouting "Only Buhari!"
In Kaduna, where a 24-hour curfew has been declared, election monitors say that shops are closing and people are fleeing to their homes through streets barricaded with burning tyres. Youths are clashing with the police and military in areas to the north and south of the city, with the security forces firing tear gas and live ammunition.
Local TV stations are reporting that the Kaduna home of Mr Jonathan's running mate, Vice-President Namadi Sambo, has been set on fire. They say the city's central prison has been attacked and inmates released.
In the central city of Jos, there is rioting in the Gangare area to the north of the city.
There are also reports of violent protests in the states of Gombe, Adamawa, Katsina and Sokoto.
And there are fears for the safety of the revered religious leader, the Sultan of Sokoto, who is now facing angry criticism over his support for President Jonathan.
To win in 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 22 million votes, compared with 12 million 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."