ABUJA—Bola Tinubu, the candidate from Nigeria's ruling party, was declared the president-elect of Africa's most populous nation, in the early hours of Wednesday, following a weekend election that was contested by the main opposition parties.

Tinubu, a former governor of Lagos State, will take over the presidency of a country grappling with Islamist insurgencies in the northeast, armed attacks, killings, and kidnappings, the conflict between livestock herders and farmers, cash, fuel, and power shortages, and perennial corruption, which opponents say Buhari's party has failed to eradicate despite promises.

According to the Independent National Electoral Commission (INEC), Tinubu received 8.79 million votes, surpassing Atiku Abubakar, the leading challenger, who received 6.98 million. Younger voters preferred the outsider Peter Obi, who received 6.1 million votes.

According to Nigerian electoral law, a candidate can win by receiving more votes than their opponents as long as they receive 25% of the vote in at least two-thirds of the 36 states plus Abuja, the federal capital, which Tinubu did.

Because of the numerous technical issues caused by INEC's adoption of new technology, opposition parties rejected the results and demanded the resignation of INEC Chairman Mahmood Yakubu on Tuesday.

Tinubu asked voters to vote for him based on his track record as Lagos State governor at the turn of the century, during which he reduced violent crime, improved traffic congestion, and cleaned up trash.

The 70-year-old has, however, appeared frail in public, slurring his speech and answering questions with platitudes, as well as skipping several campaign events, raising concerns about his effectiveness.

Obi's campaign attracted young people and urban, more educated voters who were fed up with the corrupt politics of the past, the two parties that have represented it since the end of military rule in 1999, and old men who have tended to dominate them.

The results were rejected by the opposition People's Democratic Party, Labour Party, and a smaller party.

"The results being declared at the National Collation Centre have been heavily doctored and manipulated, and do not reflect the wishes expressed by Nigerians at the polls," they said in a joint statement. The charge was rejected by INEC.

"When dissatisfied parties or candidates are dissatisfied with the outcome of an election, there are procedures for them to follow," it said in a statement.

The election was also marred by violence in some areas, though not to the same extent as previous ones.

The INEC had promised to upload results from each polling unit to its website as soon as possible, but most units were unable to do so, and thousands of results remained unpublished.

As a result, results had to be manually collated inside ward and local government counting centers, as in previous polls, which observers also criticized as a result of poor planning.