Nigeria’s opposition Labour Party is to mount a legal challenge against the presidential election victory of Bola Tinubu of the governing APC.
Labour’s Peter Obi came third in Saturday’s vote.
The opposition says the results were rigged; it is calling for fresh polls.
Just 28% of eligible Nigerian voters took part. Some could not cast their ballot because of malfunctioning machines.
Mr Tinubu won a 37% share of the votes, a lower margin of victory than previous Nigerian presidents.
European Union election observers said there had been significant shortcomings in the electoral process, adding there had been vote-buying but it was too early say how widespread this was.