The verdict of election observers is eagerly awaited
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Nigerian President Olusegun Obasanjo has surged into a formidable lead in the country's first civilian-run presidential election for 20 years.
With more than half the votes from Saturday's poll now declared, Mr Obasanjo has received more than 66%, compared with 27% for his closest rival Muhammadu Buhari.
One of the first monitoring groups to report on its findings said the election had been largely peaceful though there were "significant flaws" in some areas.
The attention now falls on Mr Buhari, and his reaction to an apparently crushing defeat. He threatened mass action if he thought there had been rigging.
Such concerns have been concentrated in the volatile southern Niger Delta region, where Nigeria's oil is produced, and where Mr Obasanjo consistently mopped up a staggering 90% of votes.
Toughest test
Mr Obasanjo - a Christian and former military ruler popular in the south-west - has long been considered the favourite to win, but Mr Buhari, a Muslim, has strong support in the largely Islamic north.
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President Olusegun Obasanjo
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The BBC's Barnaby Phillips says that the election has served to solidify the divide between the north, where strict Muslim Sharia law has been introduced, and the south, which is predominantly Christian.
But even in the north our correspondent says, General Buhari did not crush Mr Obasanjo, with the president gaining 15-20% in states with Sharia law, where he would not be expected to perform well.
The election in Africa's most populous nation is seen as the toughest test for Nigerian democracy since Mr Obasanjo's election in 1999 ended 15 years of military dictatorship.
The goal is to achieve Nigeria's first transition from one elected civilian administration to another.
Voting for state governors also took place on Saturday - and on the previous Saturday, Mr Obasanjo's Peoples Democratic Party (PDP) won a majority in parliamentary elections.
Six killed
Voting was generally orderly and peaceful on Saturday, and in most areas the turnout was high.
Muhammadu Buhari: Counting on support in north
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But in the Niger Delta, at least six opposition supporters were reported killed in a shooting at a polling station.
The American International Republican Institute said there had been significant regional variations in the conduct of the poll.
In parts of the Delta and the south-east it reported ballot box stuffing, gross falsification of results, and ballot boxes being stolen in the middle of voting.
But in other parts of the country it said Nigerians deserved praise for generally sound and well-managed elections.
A spokesman for Mr Buhari's campaign, Sam Nda-Isai, described the election as "a big joke".
He said that in areas where there had been no election, such as the southern Rivers State, official results had nevertheless been announced.
Elsewhere, official results were very different from those recorded by the local officials of Mr Buhari's All Nigeria Peoples Party (ANPP), he said.
The opposition is looking for signs of election rigging
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He accused the Independent National Election Commission (Inec) of changing the results in favour of President Obasanjo.
But an official with Mr Obasanjo's campaign, Waziri Mohammed, dismissed the allegations, saying: "We are poised to win, so we have no need to engage in fraud."
Thousands of police officers and soldiers have been deployed around the country in case there is a hostile opposition response to the result.
Final results of the landmark poll are not expected until later on Monday or on Tuesday.
More observer groups are due to give their verdicts on the elections in the coming hours.