Millions of Nigerians are due to vote in the most competitive presidential election since military rule ended.
Since 1999, Africa's most populous country has been dominated by two parties - the ruling APC and the PDP.
But this time, there is also a strong challenge from a third-party candidate - the Labour Party's Peter Obi, who is backed by many young people.
Current President Muhammadu Buhari is stepping aside after serving two four-year terms.
READ ALSO: Nigerian politician arrested with $500,000 on election eve
His All Progressives Congress (APC) is represented by former Lagos governor Bola Tinubu, while former Vice-President Atiku Abubakar is standing for the main opposition Peoples Democratic Party (PDP). There are a total of 18 presidential candidates.
The lead-up to the polls has been overshadowed by a cash shortage caused by a botched attempt to redesign the currency, leading to widespread chaos at banks and cash machines as desperate people sought access to their money.
The new notes were introduced in order to tackle inflation, and also vote-buying. On the eve of the election a member of the House of Representatives was arrested with almost $500,000 (£419,000) in cash, and a list of people he was supposed to give it to, police say.
Whoever wins will have to deal with the currency redesign, a crumbling economy, high youth unemployment, and widespread insecurity which saw 10,000 killed last year.
After the killing of a senatorial candidate on Wednesday by suspected gunmen from the separatist group, Ipob, the parliamentary election was postponed in the south-eastern Enugu East constituency.
READ ALSO: Nigeria to close land borders on election day
The election has seen a huge interest from first-time voters and young people - a third of the 87 million eligible voters are below 35 - which may lead to a high voter turn-out than the 35% recorded in 2019.
"It is my responsibility and I have seen how important it is to vote," 19-year-old first-time-voter Blessing Ememumodak told the BBC in Lagos.
Mr Obi, 61, is hoping to break up Nigeria's two-party system after joining the Labour Party last May.
Although he was in the PDP before then, he is seen as a relatively fresh face and enjoys fervent support among some sections of Nigeria's youth, especially in the south.
The wealthy businessman served as governor of the south-eastern Anambra State from 2006 to 2014. His backers, known as the "OBIdients" say he is the only candidate with integrity, but his critics argue that a vote for Obi is wasted as he is unlikely to win.
Instead, the PDP, which ruled until 2015, wants Nigerians to vote for Atiku Abubakar, 76 - the only major candidate from the country's mainly Muslim north.
He has run for the presidency five times before - all of which he has lost. He has been dogged by accusations of corruption and cronyism, which he denies.
Most of his career has been spent in the corridors of power, having worked as a top civil servant, vice-president and a prominent businessman.
Most people consider the election a referendum on the APC, which has overseen a period of economic hardship and worsening insecurity.
READ ALSO: Nigerian senate candidate shot dead and body burnt
Its candidate, Bola Ahmed Tinubu, 70, is credited with building Nigeria's commercial hub Lagos, during his two terms as governor until 2007.
He is known as a political godfather in the south-west region, where he wields huge influence, but like Mr Abubakar, has also been dogged by allegations of corruption over the years and poor health, both of which he denies.
Voting is expected to begin at 08:30 local time (07:30 GMT), though anyone in the queue before it ends at 14:30 will be allowed to vote.
Elections are also being held for 109 federal senators and 360 members of the house of representatives, with that for governors holding in March.
The Independent National Electoral Commission (Inec) has promised free and fair elections, and has dismissed talk of postponement amid fears of insecurity in parts of the country and the cash shortage that many feared might affect its preparations.
It is the first time Inec will conduct national elections using the Bimodal Voter Accreditation System (BVAS), a facial and fingerprints technology that is thought to improve transparency by making it harder for politicians to rig the process.
Final results have been announced on the third day after voting in the last two elections, but they may come in earlier this time because of BVAS, which also uploads vote results directly to the Inec website direct from polling stations.
A candidate needs to have the most votes and 25% of ballots cast in two-thirds of Nigeria's 36 states to be declared the winner
If no-one achieves that, there will be a run-off within 21 days - a first in Nigeria's history.