Election officials in Nigeria started counting ballots as polling stations closed across the country on Saturday.
Nigerians filed out to cast votes early Saturday for a new president, Federal House of Representatives and Senate.
Voting was scheduled to start by 8.30 a.m. and accreditation ended by 2.30 p.m., according to the Independent National Electoral Commission (INEC).
INEC Chairman Mahmoud Yakubu said voting in parts of southern Edo State was suspended due to some irregularities.
"There were some issues regarding violence in Esp State. We have resolved to suspend the House of Representatives election in the area. Elections there will now be held simultaneously with the governorship poll on March 11," Yakubu announced at a press conference in the nation's capital Abuja.
An election official, Suleiman Abdulsalam said voting dragged to evening time because many voters were still in the queue.
"We have instruction to accredit voters who are on the queue between 8:30 a.m. and 2:30 p.m. and allow them to vote," he told Anadolu Agency.
The success of Nigeria's vote will be closely watched in West Africa where coups in Mali and Burkina Faso and growing militancy have taken the region's democracy back a step.
Whoever wins Nigeria's presidency must manage Africa's largest economy beset by a host of complex problems, from a grinding terrorist war and bandit militias and separatists to high inflation and widening poverty.
But Saturday's vote went ahead mostly peacefully.
Several Lagos polling booths were ransacked, according to INEC, and voting at 141 polling units in southern Bayelsa State would take place on Sunday after the ballot was disrupted.
Voters will also cast their ballot for Nigeria's two houses of parliament, the National Assembly and Senate.
Buhari, a former army commander, steps down after two terms in office, with critics saying he failed in his promises to make Nigeria safer.
APC's candidate Tinubu, 70, a long-time political kingmaker, says "It's my turn" for the presidency. He can count on APC's structure and his political network.
He faces a familiar rival -- PDP candidate Abubakar, 76, who is on his sixth bid for the top job and touts his business experience.
But both are old guard figures who have fought off past corruption accusations.
The emergence of a surprise third candidate appealing to young voters, Labour Party's Obi, 61, has thrown the race open with his campaign as the candidate of change.
Fuel and cash shortages caused by a banknote exchange in the run-up to the election also left many Nigerians struggling more than usual in a country already hit by more than 20% inflation.
To win the presidency, a candidate must get the most votes, but also win 25 percent in two-thirds of Nigeria's 36 states.
If no candidate wins, a runoff will take place within 21 days between two frontrunners -- an unprecedented outcome that some analysts say is a possibility this time around.
The rules reflect a country almost equally split between the mostly Muslim north and predominantly Christian south, and with three main ethnic groups across the regions: Yoruba in the southwest, Hausa/Fulani in the north and Igbo in the southeast.
Voting also often falls along ethnic and religious lines: Tinubu is a southern Yoruba Muslim, Abubakar is an ethnic Fulani Muslim from the northeast and Obi is a Christian Igbo from the southeast.
