Nigeria's Obi disputes presidential election result

He said the election was credible and the reported problems had had no impact on the overall outcome.

The main opposition candidate, Atiku Abubakar, who came second with 29 per cent of the vote, also said later in the day he would challenge the result in court as fraudulent.

Obi, an outsider popular with young and educated urban voters, received 25 per cent, according to the Independent National Electoral Commission (INEC).

"Let me assure all Nigerians that we will explore all legal and peaceful options to reclaim our mandate," Obi, 61, told a news conference in the capital Abuja, making his first public remarks since he cast his ballot on Saturday.

"We won the election and we will prove it to Nigerians," he said.

The opposition parties said the vote had been rigged after new technology INEC had promised would make the process more transparent instead malfunctioned, eroding trust.

"The good and hardworking people of Nigeria have again been robbed by our supposed leaders," Obi said. 

He did not go into further detail on his accusations, saying evidence would be presented in court.

Obi's slick social media campaign and position as a challenger to the two parties that have alternated in power since the end of army rule in 1999 won him a devoted following among young voters calling themselves the Obidients.

There have been numerous legal challenges to the outcome of past Nigerian presidential elections but none has succeeded.

The row over the election comes as Nigeria is struggling with Islamist insurgencies, an epidemic of kidnappings for ransom, conflicts between farmers and herders, high inflation, widespread food insecurity and a shortage of cash that has caused chaos in people's daily lives.

"The weekend election was neither free nor fair," Atiku told journalists, adding he would contest the results in court.

"The processes and outcome (were) grossly flawed and must be challenged by all of us."

Both candidates questioned figures showing a low turnout at a time when there was a record number of registered voters.

INEC said almost 25 million votes were cast, out of 87 million people with voter identity cards and eligible to vote, giving a turnout rate of 29 per cent.

Tinubu was declared the winner with 8.79 million votes. 

Nigeria has a population of more than 200 million.

Election observers from the European Union and the Commonwealth reported a range of problems with the election including widespread technical failures of systems designed to prevent manipulation and improve transparency. 

They criticised INEC for poor planning but did not allege fraud.

Tinubu has already received congratulations from several foreign leaders including Prime Minister Rishi Sunak of Britain, Nigeria's former colonial ruler and one of its closest diplomatic allies.