The opposition candidate has called the provisional results in Sierra Leone "daylight robbery", alleging that his electoral agents were not allowed to verify the ballot counting.
With the majority of ballots counted, the incumbent, Julius Maada Bio, appears to have a strong lead over his main rival, Samura Kamara.
Tthe BBC's correspondent in the capital Freetown says Mr Bio's success can more likely be attributed to a series of strategic electoral alliances he made in the course of the campaign, including in opposition strongholds.
European Union observers have criticised the electoral commission for a lack of transparency, and noted incidents of violence in some regions during the vote.
The electoral commission in Sierra Leone has said it expects final results from Sunday's presidential election to be published later.
