Nigeria's two main opposition leaders on Tuesday filed petitions seeking to cancel results from last month's disputed presidential election, court papers showed, to begin what could be a legal battle lasting several months.
There have been numerous legal challenges to the outcome of previous Nigerian presidential elections, but none has succeeded.
Atiku Abubakar from the biggest opposition People's Democratic Party (PDP) and Labour Party's Peter Obi asked the Appeals Court to invalidate the election won by Bola Tinubu of the ruling All Progressives Congress (APC) Party.
The opposition leaders said in separate affidavits the election was fraught with irregularities and accused the Independent National Electoral Commission (INEC) of breaching the law by failing to use electronic machines to upload polling station results, among other criticisms.
Atiku and Obi asked for an order "canceling the presidential election" and for the INEC to conduct a fresh vote.
Tinubu has defended the election as credible.
Obi campaigned as an outsider, galvanized 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 Tinubu and Atiku, 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.
Election observers from the European Union, the Commonwealth and other groups reported a range of problems, among them failures in systems designed to prevent vote manipulation.
The observers criticized the INEC for poor planning and voting delays, but they did not allege fraud. The commission itself apologized for the technical problems during the count.
The Appeals Court has 180 days to hear and make a ruling on Obi's challenge.
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.
Violence and voter intimidation marred last month's presidential vote as well as last weekend's governorship polls. Turnout was low, despite the highest number of registered voters, at 93 million.
