[Al Ahram] "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 news hounds in the capital Abuja.Tinubu, a former Lagos governor, is set to succeed two-term President Muhammadu Buhari, who steps down in May.
He faces immense security and economic challenges in Africa's most populous country.
Almost 25 million people cast a ballot on Saturday in a vote that was largely peaceful but marked by long delays and the slow arrival of online results, angering voters and opposition parties who claim massive vote-rigging.
Obi, 61, obtained the third largest number of votes, at 6.1 million, according to results announced Wednesday -- a significant feat for an outsider in a country where two establishment parties dominate.
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.
Former vice president Atiku Abubakar, 76, of the opposition's Peoples Democratic Party (PDP), obtained the second largest number of overall votes, at 6.9 million.
Abubakar who lost his sixth bid for the presidency has not yet officially commented on the outcome of the election.
Candidates who want to submit legal challenges have 21 days following the announcement of results to bring their case to the courts.
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."