Nigerian leader Olusegun Obasanjo is waiting to be announced as the winner of the country's first civilian-run presidential election for 20 years.
The verdict of election observers is eagerly awaited
With results declared from 96% of the constituencies in Saturday's poll, Mr Obasanjo has received 61.2%, compared with 32.7% for his closest rival Muhammadu Buhari.
An official confirmation of Mr Obasanjo's victory is expected later on Tuesday, once there is verification that he has won at least 25% of votes in two-thirds of Nigeria's 36 states.
But the poll has been marred by allegations of vote-rigging.
Mr Buhari's All Nigeria People's Party (ANPP) has denounced the results as "a joke".
European Union observers said the elections were "marred by serious irregularities and fraud".
The opposition parties have been called together for a meeting in the capital to discuss their response to the results.
Mr Buhari's party had threatened mass action if the polls were rigged.
If the ANPP do reject the results, the concern is that an already tense situation could lead to unrest, says the BBC's Dan Isaacs in the northern city of Kaduna.
Much will depend on the tone of Mr Buhari's statement due later today and through his campaign team he has already called on his supporters to remain calm and not to provoke violence, our correspondent says.
There is now a high security presence in many cities across the country amid concerns that opposition supporters will react angrily to the victory of Mr Obasanjo.
The majority of foreign observers have praised the organisation of the polls across much of the country but criticised polls in the south - especially in the oil-rich southern Niger Delta.
The monitoring team from the EU gave a particularly negative assessment saying their observers "witnessed and obtained evidence of widespread election fraud in 13 states".
"The presidential and a number of gubernatorial elections were marred by serious irregularities and fraud - in a certain number of states, minimum standards for democratic elections were not met," the group said in a statement.
However, Commonwealth observers gave a more positive assessment saying: "in most of Nigeria a genuine and largely successful effort was made to enable the people to vote freely".
They added that in some states like Enugu and Rivers State "proper electoral processes appear to have broken down and there was intimidation".
The Washington-based National Democratic Institute said they had some serious concerns finding "ballot stuffing, rigging, voter intimidation, violence and fraud", particularly in the southern oil-producing delta region and the southeast.
The International Republican Institute (IRI), linked to the US Republican Party, singled out results in Cross River, Imo and Rivers states where it had found "outright or attempted fraud".
Reports are also continuing to come in of violent incidents on election day, including the reported deaths of eight opposition supporters in central Benue State and six opposition supporters in the Niger Delta during political protests.
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.
The BBC's Barnaby Phillips says the election has served to solidify the divide between the north, where strict Muslim Sharia law has been introduced, and the mainly Christian south.
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.
But what appears to have happened is that Mr Obasanjo's Peoples Democratic Party (PDP) has tightened its grip on all the political levers of power.
The PDP won 27 of the 36 state governorships in Saturday's poll - and on the previous Saturday, the PDP won a majority in parliamentary elections.