Nigeria's ruling party candidate has been declared president-elect of Africa's most populous nation in the early hours of Wednesday after a weekend election disputed by the main opposition parties.

Bola Tinubu, a former governor of Lagos state, will take over leadership of a country grappling with Islamist insurgencies in the north-east, armed attacks, killings and kidnappings, conflict between livestock herders and farmers, cash, fuel and power shortages, and perennial corruption that opponents said Muhammadu Buhari's party has failed to stamp out despite promises to do so.

The Independent National Electoral Commission (INEC) said Mr Tinubu garnered 8.79 million votes, ahead of main opposition challenger Atiku Abubakar's 6.98 million votes. Peter Obi, an outsider popular with younger voters, garnered 6.1 million votes.

Nigerian electoral law says a candidate can win by getting more votes than their rivals, provided they get 25 per cent of the vote in at least two-thirds of the 36 states and the federal capital Abuja, which Mr Tinubu did.

Opposition parties rejected the results as the product of a flawed process, which suffered multiple technical difficulties owing to the introduction of new technology by INEC, and on Tuesday called on its chairman, Mahmood Yakubu, to resign.

Mr Tinubu appealed to voters on his track record during his two terms as Lagos state governor at the turn of the century, during which he reduced violent crime, improved the city's traffic jams and cleaned up rubbish.

The 70-year-old has, however, sometimes appeared frail in public, slurring his speech, answering questions with platitudes, and skipping several campaign events, leaving some to doubt how effective he would be.

Mr Obi's campaign attracted young people and urban, more educated voters fed up with 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.

Oppositions cry foul

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

"The results being declared at the National Collation centre have been heavily doctored and manipulated and do not reflect the wishes of Nigerians expressed at the polls," they said in a joint statement.

INEC rejected the charge.

"There are laid down procedures for aggrieved parties or candidates to follow when they are dissatisfied about the outcome of an election," it said in a statement.

The election was also marred by violence in places, although not yet on the scale of previous events.

The INEC had promised to upload results from each polling unit to its website, but most units were unable to do so immediately, and thousands of results had yet to be uploaded.

That meant results had to be collated manually inside ward and local government counting centres as in previous polls, which observer missions also criticised as the result of poor planning.

Reuters