Nigerians voted on Saturday in what looks set to be the first genuine electoral contest since the end of military rule in 1999, one in which an opposition aspirant has a fighting chance of unseating the incumbent, President Goodluck Jonathan.
Seeking a second elected term, Jonathan is facing off against former military ruler Muhammadu Buhari in a tense race with an electorate divided along ethnic, regional and in some cases religious lines.
Twelve other minor candidates are also running.
Polls open across 119,973 polling stations at 8 a.m. (0700 GMT) for accreditation, then actual voting starts at 1.30 p.m. and continues until the last person has voted. With 56.7 million eligible voters, it could drag well into Sunday.
In the northern city of Kano registration started on time but elsewhere voters had to wait in the brewing tropical heat as election officials failed to turn up.
"It is 9 am now and we have not seen anybody. They were suppose to be here before we arrived but they are not," said Linus Okorie, one of 60 voters getting impatient at the Life Camp polling station in Abuja.
The start was also delayed in at least three polling stations in the southern city of Lagos.
The vote is seen as a referendum on the record of Jonathan, a former zoology professor whose time in office has been blighted by massive corruption scandals and an insurgency by Islamist Boko Haram militants in which thousands have died.
A credible and peaceful poll would open a new chapter in the history of Africa's most populous nation, biggest economy and top oil producer, whose five decades of independence have been tarnished by graft, military coups and secessionist movements.
Previous elections have been marred by rigging, and the poisonous rhetoric emanating from both sides during the campaign, as well as some scuffles and shootings, have raised fears it could be a catalyst for violence.
When Buhari, a northern Muslim, lost to Jonathan, a southern Christian and son of a canoe carver in the oil producing Niger Delta, in 2011 it triggered rioting in the mostly Muslim north that killed 800 people and rendered 65,000 homeless.
Fearing even more bloodshed this time, world leaders including U.S. President Barack Obama and British Prime Minister David Cameron have urged both sides to chill the ardour of their supporters.
In a televised address on Friday Jonathan told anybody with violent intent to "think again".
It did little to reassure many Nigerians, who on Friday were queuing up en masse to withdraw cash, buy dwindling fuel supplies and stock up on provisions at supermarkets.
"I filled my tank just in case I need to flee," said James Ike, a banker in Kaduna, the city worst affected by violence last time around. "But I hope I don't have to do that,"
In a climate of mutual suspicion, Buhari's All Progressives Congress warned against "devilish moves by those ... bent on rigging the elections and plunging the nation into crisis."
Buhari's top selling point is a belief that he didn't steal during his 1983-85 presidency.
His reputation as an army strongman also plays well with voters critical of the government's failure to protect civilians from Boko Haram, highlighted when they kidnapped over 200 school girls in April last year.
The sect has been routed in the past six weeks, though it is not clear if that will assist Jonathan, who has also made much of his clean-up of farm fertiliser subsidies and privatisation of the rotten state power provider.
But as always in this nation of 170 million people, ethnic and regional sentiments remain paramount. Buhari is hugely popular in the north, Jonathan, in the south and east.
That could leave the mostly ethnic Yoruba but religiously mixed southwest, centred around the commercial capital Lagos, the kingmakers. They voted for Jonathan last time but since then Yoruba elites have rallied decisively around Buhari.