Nigeria's opposition on Thursday bolstered their demands for the annulment of the election results, just one day after the nation declared its newly-elected president.

"We won the election and we will prove it to Nigerians," third-place Labour Party candidate Peter Obi said at a news conference in the capital. He said that the election would be remembered as one of the most controversial in Nigeria's history and claimed that it was marred by irregularities.

Hours later, second-place candidate Atiku Abubakar with the Peoples Democratic Party also rejected defeat and said he was consulting his lawyers on how to challenge the outcome.

"I have come to the conclusion that the processes and outcome of the Presidential and National Assembly election of last Saturday were grossly flawed in every (way) and as such must be challenged," he told reporters in the capital, Abuja.

At least four other parties are joining them in challenging the results. They have three weeks, from the day the final tally was announced, to appeal.

But an election can be invalidated only if it is proven that the national electoral body largely did not follow the law and acted in ways that could have changed the result. None of Nigeria’s presidential election results has ever been overturned by the country’s Supreme Court.

The opposition said the delay in uploading results from the country’s 177,000 polling stations to the electoral body's portal could have made room for vote tampering. They said there was also voter intimidation and cases where people were barred from voting at all.

While there were inconsistencies in the results in Rivers and Imo states between the information gathered by observers on the ground and the results announced by the electoral body, it was not enough to impact the election’s final outcome, said YIAGA Africa, Nigeria’s largest election observer group. Still, the issues spotted could just be the tip of the iceberg, it said.

President-elect Bola Tinubu of the ruling party received 37% of the vote share in last weekend’s election and will be Nigeria’s first president to take office with less than 50%, analysts say. The main opposition candidate, Atiku Abubakar, won 29% of the vote, while third-place finisher Obi got 25%, according to official results.

Tinubu, 70, faces a divided nation and many younger Nigerians doubt his ability to improve economic opportunities for all, let alone reduce violence and corruption in a country that is one of the world’s leading suppliers of oil.

While the opposition cried foul on Thursday, Nigerians were uncharacteristically indifferent. Unlike after previous elections where people took to the streets to celebrate or protest, the streets in Abuja were largely empty as daily life continued. Tinubu’s supporters believe he won fairly and will be sworn in as president on May 29.

"We have voted for him and he has been sworn in. It is now his turn to help us and see the way Nigeria is suffering," said Gbemisola Olabogun, a water-seller in Lagos city. "He should look into everything and make life easy for all of us."

Still, opposition supporters are holding out hope that the vote will be overturned.

"As far as I am concerned, Obi is my president," said Chima Ekwueme a Labour Party supporter. "We will be here when he is declared winner," he said.