Factbox-Nigeria election 2023: how it all works

By MacDonald Dzirutwe and Sofia Christensen

ABUJA (Reuters) - Nigerians are set to vote in presidential and parliamentary elections on Saturday that are seen as the most wide open since Africa's most populous nation switched from military rule to democracy in 1999.

Of the 18 candidates vying to succeed President Muhammadu Buhari, three stand a chance: former Lagos Governor Bola Tinubu, 70, the ruling party candidate; former Vice President Atiku Abubakar, 76, the main opposition candidate; Peter Obi, 61, a challenger popular among young voters.

Here are key facts about the logistics of the election, Africa's largest democratic contest:

WHEN IS IT?

Voting takes place from 8 a.m. to 2 p.m. (0700 GMT to 1300 GMT) on Feb. 25.

Voters who join a polling station queue on time will be allowed to cast their ballots after closing time.

HOW BIG IS IT?

More than 93 million people are registered to vote.

There will be about 176,600 polling stations across the country, including in camps for people displaced by conflict between Islamist insurgents and federal troops in the northeast.

WILL IT BE CLEAN?

The Independent National Electoral Commission (INEC) says it has taken measures to ensure this election will be free and fair - a major concern in a nation with a long history of electoral fraud and violence.

Voters will present their voting cards and be identified by a Bimodal Voter Accreditation System (BVAS) that uses fingerprints and facial recognition. This is aimed at reducing the risk of fraudsters voting several times.

On voting day, results from individual polling stations will be pasted outside for citizens to view, and sent through BVAS to an INEC portal. The results will be displayed on the portal in real time and accessible by the public, INEC says.

It is hoped this will reduce the risk of the figures being manipulated along the way.

INEC says that citizens wishing to witness the sorting and counting of ballot papers at polling stations should do so from a distance of 300 metres. In the past, one of the forms of vote-rigging seen in Nigeria was the intimidation of electoral officials by thugs paid by politicians.

WHAT DO CANDIDATES NEED TO WIN?

A presidential candidate must receive the largest number of votes cast nationwide and at least a quarter of the vote in no fewer than 24 of Nigeria's 36 states.

If no one clears both hurdles, the top two candidates will compete in a run-off. The constitution says the run-off must take place within 21 days of the announcement of the result.

No run-off has been necessary since the transition to democracy in 1999.

For the parliamentary poll, candidates for a seat in the House of Representatives or the Senate must win a simple majority of votes in the constituency or senatorial district they are contesting in order to win.

WHEN WILL RESULTS BE ANNOUNCED?

The counting and collating process is expected to take several days.

While INEC has not said exactly when it would declare official results, this is expected to take place in the first half of next week.

(Editing by Estelle Shirbon and Alex Richardson)