Nigeria's opposition presidential candidate Peter Obi, who came third in last month's election, has filed a formal petition in court seeking to overturn the ballot won by the ruling party candidate.
The petition dated Monday is likely the first stage in a long legal battle over the February 25 election, as has happened with past presidential races in Africa's most-populous nation.
The Labour Party's Obi was a surprise third candidate whose appeal to younger voters allowed him to challenge the dominance of the ruling All Progressive Congress (APC) and the main opposition Peoples Democratic Party (PDP).
Former Lagos governor and APC stalwart Bola Tinubu won the election with around 37 percent of vote, but opposition parties say delays in electronic uploading of results aided massive ballot rigging.
In the petition filed in an Abuja appeals court, Obi alleges that the Independent National Electoral Commission or INEC broke electoral law.
INEC has denied any illegal activity, though acknowledged technical problems.
Among other claims, the petition says Tinubu was not qualified to be a candidate because of a 1990s drug-related forfeiture of nearly $500,000 from one of his accounts in a US bank. Tinubu denies any wrongdoing.
Nigeria's ruling APC won most of the states in the local elections, but on the back of a low turnout.
"The election... was invalid by reason of corrupt practices and non-compliance with the provisions of the electoral act," the petition says.
It claims "based on the valid votes cast", Obi won the largest number and "ought to be declared and returned the winner of the presidential election".
The main opposition presidential candidate PDP's Atiku Abubakar has also said he will challenge the results, calling the election a "rape of democracy".
Analysts expect those legal challenges to end up the country's Supreme Court, as they did after the 2019 election.
President Muhammadu Buhari steps down in May after two terms, leaving Nigeria grappling with widespread insecurity, economic woes and growing poverty.
Nigerians had hoped the presidential ballot would give them a chance to be heard, but many were disappointed by the way the election was conducted.
Voters and opposition parties complained last month that technical mishaps with voting machines caused delays and allowed for vote rigging, which the electoral commission has denied.
International observers, including from the European Union, noted major logistical problems, disenfranchised voters and a lack of transparency.
"The process of reclaiming the people´s mandate has started," Labour spokesman Yunusa Tanko said in a statement on Tuesday.
Earlier on Tuesday, president-elect Tinubu appealed for unity, saying "the time for politicking is gone".
- Violent intimidation -
Nigeria's ruling party also won the majority of governorships contested in last weekend's local elections, results showed Tuesday, following a ballot marred by voter suppression and violent intimidation which the US government called deeply troubling.
Elections were held to choose governors in 28 of Nigeria's 36 states and state assembly lawmakers. Governors in the remaining eight states had earlier been chosen in by-elections.
According to the results from Saturday's vote declared by INEC, APC won the governorship in 15 states -- Lagos, Sokoto, Katsina, Jigawa, Gombe, Kwara, Niger, Yobe, Nasarawa, Cross River, Ebonyi, Ogun, Benue, Kaduna and Borno.
PDP won seven -- Plateau, Bauchi, Oyo, Delta, Rivers, Akwa Ibom and Zamfara states.
Governors are powerful figures in Nigeria and some control state budgets that are larger than those of several African nations.
In a major upset in the presidential election, outsider Obi won the most votes in Lagos, considered the fiefdom of president-elect Tinubu.
A big question on Saturday was whether Obi's popularity, especially among younger voters, would translate into success at the local polls.
But the APC's Babajide Sanwo-Olu scored a landslide re-election as Lagos governor.
Local and international observers said the latest poll was impacted by disappointments in the presidential election but also by tactics to scare voters, buy ballots and threats of violence.
"The United States is deeply troubled by the disturbing acts of violent voter intimidation and suppression that took place during the March 18 polls in Lagos, Kano, and other states," the US embassy said in a statement on Tuesday.
EU mission Chief Observer Barry Andrews on Monday said Nigerians' expectations for Saturday's election were not met in many parts of the country.
"Many were disappointed and we witnessed voter apathy that is a clear consequence of failures by political elites and, unfortunately, also by INEC," he said.
His mission also said polling was disrupted by "thuggery and intimidation of voters, polling officials, observers, and journalists."
"Unfortunately, there were many casualties and fatalities," it said. "Vote-buying, also directly observed by EU (election) observers, further detracted from an appropriate conduct of the elections."