The candidate who came third in Nigeria's presidential elections has vowed to prove in court that his supporters were "robbed" of victory.
Labour Party's Peter Obi, who gained 25% of the votes according to official results, was speaking in public for the first time since the weekend poll.
"We won the election and we will prove it to Nigerians," he said.
Bola Tinubu of the ruling APC party was declared the winner, with 37% of the vote.
READ ALSO: Nigeria poll: Peter Obi plans to challenge Tinubu's victory
Atiku Abubakar of the main opposition PDP came second with 29%.
Mr Obi said Saturday's elections would go down as "one of the most controversial elections in Nigeria's history".
"The good and hard-working people of Nigeria have again been robbed by our supposed leaders whom they trusted," he told journalists on Thursday.
He has 21 days from the day the results were announced to challenge the result at Nigeria's highest appeal court.
Mr Obi refused to say what evidence he had to prove that he won the election but his challenge is likely to be based on the inability of the Independent National Electoral Commission (Inec) to publish results from the polling units in real time using a new electronic transmission system.
Inec officials had promised that results from each of the more than 176,000 polling stations would be scanned and uploaded to its website to aid transparency.
But that was not possible on Saturday, with Inec apologising for what it called "technical glitches". It said that any discrepancies between results on the portal and "physical results" would be investigated and resolved.
Inec also blamed delays on factors beyond its control, like the lack of internet access at some polling stations, for its inability to upload the results.
But days after the elections ended, the commission has still not uploaded all the results from the polling units to its website.
The elections were marred by lengthy delays which saw some voters queuing through the night to cast their ballots and voting extended to Sunday in some areas.
There were also some cases of arming men attacking polling stations and stealing ballot boxes.
But it is not clear whether such problems were enough to affect the outcome, as Mr Tinubu had a margin of victory of about 1.8 million votes.
Although Mr Obi was with the PDP until last year, and was its vice-presidential candidate in 2019, he was seen by many as a fresh face, and mounted a strong challenge to the two parties which have dominated Nigeria since the end of military rule in 1999.
He gained particular support among young people, especially in the south, who feel they have been failed by previous generations of political leaders.
To win in the first round, a candidate must have the largest number of votes nationwide and at least 25% of the votes in two-thirds of the 36 states and Federal Capital Territory (Abuja)
BBC