His main rival, runner-up Atiku Abubakar, has called the election a "sham" and vowed to challenge the result in court.

Here are five things we've learnt from an election marred by controversy.

1. It broke records - for the wrong reasons

With 73 million able to vote, this could have been Africa's biggest-ever election - but only a third of the electorate showed up.

So what was billed as a record-breaking election did break the records but for an altogether different reason. The 2019 general election recorded the lowest turnout in Nigeria's 20-year history as a democracy.

Nationwide turnout has been on a steady decline since 2003. The general decline - especially in the south - could indicate a decreasing faith in the political establishment and what it can deliver for the people. Voter apathy appears to have set in.

2. Buhari is still a big deal in the north

The runner-up and main opposition candidate, Atiku Abubakar, says there was foul play in the tallying of votes.

He says it is curious that the total number of votes cast in one of his strongholds, Akwa-Ibom, was 50% lower in this election than it was in 2015.

Mr Buhari normally has significant support in the north of the country, where he is seen as a principled man of the people. His numbers have been consistent there for the past five presidential elections.

Turnout was significantly lower in Nigeria's southern regions, where Mr Abubakar had hoped his numbers would increase.

In a press statement on Wednesday, he specifically mentioned the turnout in Akwa Ibom, which was only 29%. Mr Abubakar said it was much higher in the 2015 election and put the difference down to "voter suppression".

He did win in the south, but not by a big enough margin to cut Mr Buhari's lead of four million votes.

3. Security concerns did not deter Buhari supporters

Mr Abubakar also questioned why the parts of the north ravaged by the Islamist militia group, Boko Haram, had high voter turnouts.

Borno and Yobe states in the north-east are strongholds of the governing All Progressives Congress (APC). Their support has not wavered, even though security concerns have displaced some two million people.

The Independent National Electoral Commission (Inec) made provision for the 400,000 internally displaced people to vote in or around their camps.

The region delivered high numbers despite attacks on election day by Boko Haram and its offshoot, the so-called Islamic State West Africa Province (Iswap).

No more presidential election posters for another four years

But there was violence in parts of the south of the country including in Rivers state. As a consequence voting did not take place in two local government areas and could have affected turnout in the state as a whole, Idayat Hassan, from the Abuja-based Centre for Democracy and Development, told the BBC's Newsday programme.

Again, this is likely to have affected Mr Abubakar's overall vote count, as he won 74% of the votes which were cast in the state.

4. Both candidates were affected by the delay

Inec delayed the election day by a week because of logistical problems.

This sparked complaints from people who had already travelled to their home towns to vote, and now would have to make the journey twice.

However, this would have affected the supporters of both candidates equally.

5. Electronic voting is - probably - more transparent

Some analysts suggest the use of electronic voting systems has made human error and manipulation more difficult.

Though there are reported cases of electronic voter verification devices failing, many believe the use of technology has helped understand the electorate and their true behaviour at the polls, as well as curb voter fraud.