Partial results from Nigeria's election give ex-military ruler Muhammadu Buhari a narrow lead over the incumbent, President Goodluck Jonathan.
With more than half of Nigeria's 36 states declared, Gen Buhari's All Progressives Congress (APC) is ahead by half a million votes.
A victory for Gen Buhari would make President Jonathan the first incumbent to lose an election in Nigeria.
Correspondents say it is likely the loser will allege foul play.
More than 800 people were killed in protests after Mr Jonathan beat Gen Buhari in the previous election.
Gen Buhari's lead was cut from two million votes, after Mr Jonathan gained a landslide in Rivers State, where there have been widespread reports of irregularities and a curfew imposed.
Electoral commission (Inec) chairman, Attahiru Jega, said the fact-finding team sent to the state found there were some voting irregularities with the poll but not enough "to warrant a cancellation of the election".
However, many of the states still to declare are in the north, where Gen Buhari is seen as favourite.
Earlier, the announcement of results was disrupted when an agent of Mr Jonathan's People's Democratic Party (PDP) launched a tirade against Mr Jega in Abuja.
"We have lost confidence in what you're doing, we don't believe in you any more," Elder Orubebe said.
Rejecting the allegation, Mr Jega replied: "Let us be careful about what we say or do and let us not dispute a process that has begun peacefully."
Gen Buhari has won the two biggest states, Lagos in the south and Kano in the north, while Mr Jonathan won a huge majority in his home state of Bayelsa, as well as Rivers.
The candidate with the most votes will only avoid a run-off if they gain at least 25% of the votes in two-thirds of Nigeria's 36 states.