European Union observers have said Nigeria's presidential elections have been marred by serious irregularities and fraud.
"The presidential and a number of gubernatorial [governorship] elections were marred by serious irregularities and fraud - in a certain number of states, minimum standards for democratic elections were not met," the 118-strong EU observer team said in a statement.
"[The] observers witnessed and obtained evidence of widespread election fraud in 13 states," the statement said. With votes counted in 725 of 774 areas, Nigerian leader Mr Olusegun Obasanjo - a born-again Christian from southern Nigeria - had an unassailable lead with 21.9 million ballots, or 61.3 per cent.
Rival Mr Muhammadu Buhari, from the predominantly Muslim north, had 11.7 million votes or 32.6 per cent. The other 18 candidates vying to lead the country of 120 million shared the rest.
The EU's verdict overshadows a report from the Commonwealth that had boosted Mr Obasanjo, a prominent player in efforts to attract investment to Africa in return for better governance and fair elections.
At least four independent groups of foreign and local poll observers have listed serious electoral fraud affecting the results in at least seven southern states.
Nigerians are hoping to end decades of instability and improve their country's poor international image by successfully transferring power from one elected government to another.
More than 10,000 people have been killed in ethnic, religious and political clashes in Nigeria since a 1999 military-supervised election that put Mr Obasanjo in office.