Nigeria's opposition Labour Party candidate Peter Obi has filed a court petition challenging last month's disputed presidential election, the party said, kicking off what could be a long legal campaign lasting several months.
There have been numerous legal challenges to the outcome of previous Nigerian presidential elections but none has succeeded.
Obi campaigned as an outsider, galvanised young and first-time voters and had appeared to throw the contest wide open, raising some voters' hopes for change after years of hardship and violence under outgoing President Muhammadu Buhari, 80, a former army general.
But Obi came third behind winner Bola Tinubu of the ruling All Progressives Congress party (APC) and main opposition People's Democratic Party's (PDP) Atiku Abubakar, both of whom had powerful political machines and decades of networking behind them.
The APC and PDP have, between them, governed Nigeria since the end of military rule in 1999.
"We are challenging the qualifications of the candidate that was declared the winner. We are also challenging the processes that led to his declaration as the winner, among others," Labour Party spokesperson Yunusa Tanko told Reuters.
Range of problems
Election observers from the European Union, the Commonwealth and other bodies reported a range of problems, among them failures in systems designed to prevent vote manipulation.
The observers criticised the electoral commission for poor planning and voting delays, but they did not allege fraud. The commission itself apologised for the technical problems during the count.
The Appeals Court will sit as a tribunal and has 180 days to hear and make a ruling on Obi's challenge. Atiku has also said he would petition the court and has until midnight on Wednesday to file his papers.
If a candidate is not satisfied with the outcome of the tribunal, they can approach the Supreme Court, which will deliberate on an appeal within 60 days.
Nigeria's next president will be sworn in on May 29.
There were several cases of violence and voter intimidation in last month's presidential vote as well as this weekend's governorship polls.
Turnout was low despite the highest number of registered voters, at 93 million.
The APC won 15 of the 28 governorship races, including commercial hub Lagos, compared to PDP's 8 and one went to a regional northern party.
Elections in two states were declared inconclusive and counting was suspended in two others due to violence.