Nigeria's presidential candidate Peter Obi, who lost a historically tight February 25 election to the ruling party's Bola Tinubu, said Thursday he would challenge the result in court.
"We will explore all legal and peaceful options to reclaim our mandate. We won the election and we will prove it to Nigerians," the Labour Party candidate told reporters in the capital Abuja.
Obi obtained the third largest number of votes, at 6.1 million -- a significant feat for an outsider in a country where two establishment parties dominate.
On Wednesday, Tinubu of the ruling All Progressives Congress (APC) was declared winner with a total of 8.8 million votes and the required number of votes across two-thirds of Nigeria's states -- a rule meant to ensure broad representation.
The former Lagos governor will succeed President Muhammadu Buhari who will step down in May.
Atiku Abubakar of the opposition's Peoples Democratic Party (PDP) obtained the second largest number of total votes, at 6.9 million.
The election in Africa's most populous nation was largely peaceful but it was marked by long delays and the slow arrival of online results, angering voters and opposition parties who claim massive vote-rigging.
"It will go down as one of the most controversial elections ever conducted in Nigeria," Obi said.
"The good and hardworking people of Nigeria have again been robbed by our supposed leaders whom they trusted."
Nigerian elections have often been marked by fraud allegations and violence.
Tinubu on Wednesday called on his rivals and their supporters to "join hands" with him, urging them to "come in so that we may begin the task of rebuilding our national home together".
Candidates who want to contest the election have 21 days following the announcement of results to bring their case to the courts.