Nigerians have begun voting to elect a successor to President Muhammadu Buhari, with many hoping the next leader will steer Africa's most populous nation and biggest economy on a new course after years of worsening violence and hardship.

Polling stations were scheduled to open at 8.30am (1830 AEDT) on Saturday, though Reuters reporters at several locations across the country saw some that were not ready.

The main candidates in the most wide-open contest since Nigeria switched from army rule to democracy in 1999 are two political veterans from the two main parties and a candidate from a minor party who opinion polls suggest has a chance thanks to support from young voters.

Buhari, a retired army general, is stepping down after serving the maximum eight years allowed by the constitution but failing to deliver on his pledge to bring back order and security across Nigeria, Africa's top oil-producing nation.

More than 93 million people are registered to vote for the next president and members of the National Assembly.

Some 176,600 polling stations were scheduled to be open. In past Nigerian elections, voters in some areas have complained that polling stations opened hours late or did not materialise at all.

The final tally from the 36 states and federal capital Abuja is expected within five days of voting.

The run-up to the vote has been marred by violence, a pattern seen in previous Nigerian elections, with the killing of a senatorial candidate in the volatile southeast region on Wednesday the latest in a series of serious incidents.

The election comes as Nigerians are struggling to cope with a shortage of cash caused by a botched plan to swap old bank notes for new ones that has wreaked havoc on people's daily lives and led to scenes of violence at banks and cash machines.

The new president will also have to grapple with problems ranging from high inflation, deep poverty and energy shortages, to an Islamist insurgency in the northeast, industrial-scale oil theft in the south and rampant crime everywhere.

The main contenders in the race to succeed Buhari are former Lagos governor Bola Tinubu, 70, of the ruling All Progressives Congress, former vice-president Atiku Abubakar, 76, of the main opposition Peoples Democratic Party, and former Anambra State governor Peter Obi, 61, of the smaller Labour Party.

Tinubu and Atiku, as he is known in Nigeria, are both political heavyweights with decades of networking behind them and bulging campaign coffers. Both Muslims, Tinubu is an ethnic Yoruba from the southwest and Atiku is a Fulani from the northeast.

Obi, a Christian from the Igbo ethnic group, has less of a political machine behind him but has used a slick social media campaign to generate huge enthusiasm among young voters.

Nigeria has a long history of electoral fraud and violence, though its polls have been getting gradually cleaner in recent cycles. The presidential candidates and parties pledged on Wednesday to support a peaceful and transparent process.