Nigeria's two main opposition parties have called for the annulment of what they have described as a “sham” election, denouncing the "massive manipulation" of the results.
A partial count has given a slight lead to ruling party candidate and former Lagos State governor, Bola Tinubu.
Saturday’s vote was generally peaceful, although there were some security incidents and logistical hitches.
But following delays in the count, and major failures in the electronic transfer of results, the opposition accused the ruling All Progressives Congress party of fraud.
Several observers have also expressed concerns about the transparency of the vote and the operational failures.
After a pause, the Electoral Commission (Inec) was due to resume announcing the results on Tuesday afternoon.
But the People's Democratic Party and the Labour Party say they have lost confidence in the process and have called for a new election to be held.
The vote was always expected to be Nigeria’s closest presidential poll in the country’s history.