Goodluck Jonathan has won Nigeria's presidential election, according to final figures announced by the Independent National Electoral Commission (INEC).
Mr Jonathan won 22.5m votes, around 57% of votes cast, compared to 12.2m votes, around 31%, for his nearest rival Muhammadu Buhari.
Violent protests erupted across northern Nigeria as youths angered at President Jonathan's victory torched buildings and set up burning barricades.
Observers have called the poll the fairest in decades in Africa's most populous nation but Mr Buhari's supporters accuse the ruling party of rigging.
Results show how politically polarised the country is, with Mr Buhari sweeping states in the Muslim north and Mr Jonathan winning the largely Christian south.
Authorities in the northern state of Kaduna have imposed a 24-hour curfew after protestors set fire to the residence of Vice President Namadi Sambo in the town of Zaria and forced their way into the central prison, releasing inmates.
'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 Zaria whose origins are in the south.
Plumes of smoke have been seen rising into the air in parts of the state capital after protestors set fire to barricades made of tyres.
Security forces have used teargas to disperse groups of youths shouting 'We want Buhari, we want Buhari'.
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.
Mr Peterle called for restraint in northern Nigeria and said all Nigerians should respect the election process.