Security forces fired in the air and used teargas across Nigeria's largely Muslim north on Monday to try to quell protests over the election victory of President Goodluck Jonathan.
The vote count showed Jonathan, from the southern oil-producing Niger Delta, had beaten Muhammadu Buhari, a former military ruler from the north, in the first round.
Observers have called the poll the fairest in decades in Africa's most populous nation but Buhari's supporters accuse the ruling party of rigging. Results show how politically polarised the country is, with Buhari sweeping states in the Muslim north and Jonathan winning the largely Christian south.
Plumes of smoke rose into the air in parts of the northern city of Kaduna as protesters set fire to barricades of tyres. Groups of youths shouted "We want Buhari, we want Buhari".
Residents in the town of Zaria said a church was burned overnight and soldiers dispersed scuffles between rival supporters at the gates of the Emir's palace.
"They have destroyed our cars and our houses. I had to run for my life and I am now in my neighbour's house," said Dora Ogbebor a resident of the town of Zaria whose origins are in the south.
Soldiers used whips to disperse people gathering in the streets of Kano, the most populous city in the north. Gunfire broke out in one neighbourhood and protesters hurled stones.
An armoured personnel carrier, armed police and soldiers formed a barricade around the electoral commission office.
"We will have the situation under control soon," said Agbo Omaji, a police inspector securing the electoral office.
Soldiers fired in the air and helicopters flew overhead in the central city of Jos, where thousands have been killed in sectarian violence over the past decade.
Nigeria has a history of rigged and violent elections but Saturday's vote was deemed by many Nigerians, and foreign observers, to have been a vast improvement on the past, with the voting process orderly and little unrest on the day itself.
"Election day showed a generally peaceful and orderly process," said chief European Union election observer Alojz Peterle. EU observers said 2007 elections were not credible.
Peterle called for restraint in northern Nigeria and said all Nigerians should respect the election process.
A Reuters tally of results put Jonathan on nearly 23 million votes to just over 12 million for Buhari. The Independent National Electoral Commission was expected to announce all the results on Monday and to formally declare Jonathan the winner.
The outright win for Jonathan could ease worries over potential disruptions to crude exports from Africa's biggest oil and gas industry -- far away from the disturbances in the north.
It could also lift local financial markets which had been unnerved by the prospect of a potential run-off and the All-Share Index was up over 2 percent in early trade to its highest in nearly a month.
But Buhari's camp -- which had urged its supporters throughout the campaign to make sure their votes counted -- said some results looked suspicious, especially where turnout had been exceptionally high in some of Jonathan's strongholds.
"In most of the southeast and south-south, no real elections took place," former government minister Nasir el-Rufai, a Buhari supporter, told Reuters late on Sunday.
"In the southwest and the north, the results have no relation to what happened at the polling units and we will prove it in due course," he said.
Buhari, who also lost elections in 2003 and 2007, has repeatedly said Nigerians would not accept another rigged vote. He told Reuters on Saturday he would not go to court to challenge the outcome but that his party may chose to do so.