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.
There are unconfirmed reports that people have died in the violence.
President Jonathan has called for an end to the disturbances, saying "no-one's political ambition is worth the blood of any Nigerian".
With nearly all the votes counted, People's Democratic Party (PDP) candidate 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.
A lawyer travelling through Kaduna told the BBC's Focus on Africa he had escaped from a mob in the city. He said youths armed with clubs and machetes were targeting people who did not look like they were indigenous to the north.
"My car was damaged [and] the windscreen was broken," he said. "I told my driver... to start the car and take off and at that point they smashed the car. We managed to get away."
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 as well as a majority of the total votes cast.
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%.
They accuse PDP politicians in the north of rigging the vote, while in some areas of the south they say there is a discrepancy between turnout and results.
"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."
To win at the first round, a candidate not only needs the majority of votes cast, but at least 25% of the vote in two-thirds of Nigeria's 36 states. Goodluck Jonathan, of the PDP, reached that threshold in 31 states; runner-up Muhammadu Buhari of the CPC only did so in 16 states.
Nigeria's 160 million people are divided between numerous ethno-linguistic groups and also along religious lines. Broadly, the Hausa-Fulani people based in the north are mostly Muslims. The Yorubas of the south-west are divided between Muslims and Christians, while the Igbos of the south-east and neighbouring groups are mostly Christian or animist. The Middle Belt is home to hundreds of groups with different beliefs, and around Jos there are frequent clashes between Hausa-speaking Muslims and Christian members of the Berom community.
Despite its vast resources, Nigeria ranks among the most unequal countries in the world, according to the UN. The poverty in the north is in stark contrast to the more developed southern states. While in the oil-rich south-east, the residents of Delta and Akwa Ibom complain that all the wealth they generate flows up the pipeline to Abuja and Lagos.
Southern residents tend to have better access to healthcare, as reflected by the greater uptake of vaccines for polio, tuberculosis, tetanus and diphtheria. Some northern groups have in the past boycotted immunisation programmes, saying they are a Western plot to make Muslim women infertile. This led to a recurrence of polio, but the vaccinations have now resumed.
Female literacy is seen as the key to raising living standards for the next generation. For example, a newborn child is far likelier to survive if its mother is well-educated. In Nigeria we see a stark contrast between the mainly Muslim north and the Christian and animist south. In some northern states less than 5% of women can read and write, whereas in some Igbo areas more than 90% are literate.
Nigeria is Africa's biggest oil producer and among the biggest in the world but most of its people subsist on less than $2 a day. The oil is produced in the south-east and some militant groups there want to keep a greater share of the wealth which comes from under their feet. Attacks by militants on oil installations led to a sharp fall in Nigeria's output during the last decade. But in 2010, a government amnesty led thousands of fighters to lay down their weapons.