LAGOS, Feb 24 (Reuters) - Voters in Africa's biggest economy and most populous nation head to the polls on Saturday to elect a new president and lawmakers, in what is set to be a close contest amid rising frustration over cash and fuel shortages, unprecedented insecurity and surging inflation.

President Muhammadu Buhari, who will complete his constitutionally allowed two terms in May, is not on the ballot. Voters will also choose new senators and members for the House of Representatives. Governorship races follow on March 11.

Here is what you need to know about the election.

WHO IS RUNNING?

A total of 18 candidates are vying for the presidency. But the main contest is between Labour Party's Peter Obi, who is leading in some polls after galvanising young voters, and veteran candidates Bola Tinubu from the ruling All Progressives Congress (APC) and Atiku Abubakar of the main opposition People's Democratic Party (PDP).

A lack of reliable polling makes it difficult to predict the winner, but a close race is expected even though the ruling party has a major advantage as it is able to use the state apparatus to mobilise support.

Tinubu and Atiku have significant powerbases across Nigeria, while Obi is banking on frustration over the economy and insecurity to turn voters against the two major parties.

WHAT ARE THE MAIN ISSUES?

Africa's top oil producer is a key Western ally in the fight against Islamist insurgents in west Africa.

The top issue for many Nigerian voters is spreading insecurity, from kidnappings for ransom in the northwest to a 13-year Islamist insurgency in the northeast, separatist violence in the southeast and decades-old ethnic tensions mostly between herders and farmers in the north central region.

Double-digit inflation is at its highest in nearly two decades, and a shortage of cash has angered Nigerians who say life is harder than when Buhari took office in 2015.

The naira currency plunged to record lows as unprecedented oil theft knocked crude exports last year, and endemic corruption remains a scourge.

As the economy suffers, hundreds of Nigerians are leaving the country in a punishing brain drain that is stretching a weak healthcare system and disrupting services from banking to tech.

WHAT DO THE PARTIES OFFER?

There are no clear ideological differences between the two major parties. Competition for dwindling oil revenues, patronage and ethnic rivalries typically play a bigger role in Nigeria's elections than ideology.

Obi, who left the PDP last year and was Atiku's running mate in 2019, casts himself as a reformist willing to overhaul Nigeria's political system. But on policy, there is little separating the main candidates. Tinubu, Atiku and Obi have all made reviving the economy and ending insecurity top priorities, promising better pay for security forces and more military equipment to defeat insurgents.

Their manifestos say they would scrap a fuel subsidy that cost $10 billion last year but differ on how quickly they would do it. They also promise to reform the forex market and invest more in education.

HOW WILL THE ELECTION WORK?

Some 93.4 million people have registered to vote, of whom three-quarters are between the ages of 18 and 49.

The challenge for the parties, however, will be getting out the vote. Many younger Nigerians do not relate to the two major-party candidates, who are both septuagenarian political veterans. In 2019, voter turnout was 35%, electoral commission figures showed.

Nigeria has a long history of electoral fraud.

This year, however, the Independent National Electoral Commission (INEC) is using a Bimodal Voter Accreditation System (BVAS) to identify voters through fingerprints and facial recognition, hoping this will curb rigging.

On voting day, results will be pasted outside polling stations and sent through BVAS to an INEC portal in Abuja. They will be displayed on the portal in real time and the public can view them.

Official results are expected within five days. The candidate with the most votes will be declared the winner if they have at least one-quarter of the vote in two-thirds of Nigeria's 36 states and the capital. Otherwise there will be a run-off between the two top candidates within 21 days.

Reporting by MacDonald Dzirutwe; Editing by James Macharia Chege, Gareth Jones and Alex Richardson

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