The election will be the largest democratic exercise on the continent as Africa's most populous nation picks a new President.

Aside from the presidential vote, the public will also be choosing their representatives for Parliament - the National Assembly- and there are 469 legislators, made up of 109 Senators and 360 members of the House of Representatives.

There are 176,606 polling stations spread across Nigeria and voting starts at 8:30 a.m. and ends at 2:30 p.m. but voters who will still be in the queue at the close of the polls at 2:30 p.m. will be allowed to slip their ballot into the ballot box.

To be declared the winner of the presidential election, a candidate must have obtained the highest number of votes and at least 25 per cent of the ballots in at least two-thirds of the 36 states and in Abuja.

If none of the candidates qualifies, there will be a run-off between the top two candidates within 21 days, which would be a first in Nigeria's democratic history.

18 presidential candidates

Eighteen candidates are in the running for Nigeria's highest office, each confident of turning the country's fortunes around if voted into power, but opinion polls suggest three are leading the race for the popular vote.

One of the key leading contenders is former Governor of Lagos, Bola Ahmed Tinubu, the candidate of President Buhari's party, the All Progressives Congress (APC); another is the main opposition leader and former Vice-President Atiku Abubakar of the People's Democratic Party (PDP), who is running for the sixth time following five previous losses, and Peter Obi, a wild-card candidate who defected from the PDP to the smaller Labour Party and now leads in at least five opinion polls.

Three leading candidates

Tinubu, 70, a former governor of Nigeria's wealthy Lagos State, wields significant influence in the south-western region where he is acclaimed as a political godfather and kingmaker and the affluent political veteran boasts of aiding the election of Buhari to the presidency on his fourth attempt in 2015, after three previous unsuccessful bids.

After decades as a political puppet master, Tinubu declared that it was now his turn to emerge from the shadows into the presidency.

His campaign slogan is, "Emi Lokan," which translates to "It is my turn," in his native Yoruba language.

Abubakar, 76, who served as Vice-President from 1999 to 2007, is a staunch capitalist who made his fortune investing in various sectors of the country.

The tycoon had been investigated for corruption in the past. However, he had denied any wrongdoing.

Presidential ambition

Many believe Abubakar's presidential ambition might usurp an unofficial arrangement to rotate the presidency between Nigeria's northern and southern regions, since he is from the same northern region as the outgoing leader, Buhari.

Peter Obi is a two-time former governor of Anambra State who is being touted as a credible alternative to the two major candidates.

Obi eschews the excesses of the typical

'African Big Man' leader.

He shuns a large entourage, flies economy class and carries his own luggage. His "no frills" approach has attracted hordes of supporters, mostly young Nigerians who call themselves 'Obidients.'

Obi is also the only Christian among the leading candidates. His south-eastern region is yet to produce a President or Vice President since Nigeria returned to civil rule in 1999.

The crucial election comes as the country battles myriad economic and security problems that range from fuel and cash shortages to rising terror attacks, high inflation and a plummeting local currency, and for the first time since the country's return to democratic rule in 1999, none of the candidates is an incumbent or a former military leader.

Nigeria's presidential elections have typically been between the ruling party and opposition party, but this year's vote has a third strong contender, Peter Obi, who is running under the lesser known Labour Party.

The past two elections have been postponed at short notice and there are fears this one will suffer the same fate.

However, the Electoral Commission insists there will be no disruptions.

Although more than 93 million Nigerians are registered to vote, uncertainty hangs over voter turnout on polling day, with insecurity among the biggest concerns.

Citizens have also been disrupted by an attempt to curb vote buying by making the old currency notes useless to prevent rogue politicians from stockpiling cash.

But, there are fears that shortage of the new naira notes could disrupt the elections itself.

The electoral body, INEC, reportedly warned that the inability of banks to distribute enough of the new cash could make it difficult to pay temporary staff and security guards needed to operate thousands of polling stations for presidential and parliamentary elections on February 25.

Winner of polls

Whoever Nigerians choose to succeed

President Buhari - only the second incumbent in Nigerian history to bow out willingly after serving two democratic terms - will have to resolve a litany of crises that had worsened under the retired army general's administration.

These include banditry and militant violence now affecting most parts of the country, systemic corruption that deters investment and enriches a well-connected elite; high inflation and widespread cash shortages after a botched introduction of new bills late last year.