Suspected Boko Haram extremists have attacked polling stations and destroyed election material in two north-eastern towns as Nigerians continue to vote.
Fleeing residents said scores of the extremists are advancing on Nigeria's north-eastern city of Bauchi, with soldiers exchanging heavy gunfire with them.
Fighter jets flew above the city and police spokesman Haruna Muhammad said security forces had stopped a convoy of 10 vehicles of "unidentified gunmen" at Dindima village, six miles from Bauchi.
Residents said the convoy is carrying Boko Haram fighters who first struck at about 7.30am at Alkaleri, a town 75 miles east of Bauchi.
Mr Muhammad said the gunmen had attacked polling stations in Kirfi and Alkaleri towns earlier.
Boko Haram extremists killed at least 41 people, including a local official, and scared hundreds of people from polling stations in three states in the north-east on Saturday.
In electoral violence elsewhere, three people including a soldier were shot and killed in southern Rivers state, and police said two car bombs exploded at polling stations in the south-east, but no-one was injured.
Elections were extended into Sunday in about 300 of the country's 150,000 polling stations, including some areas of Lagos, Nigeria's megacity of 20 million on the Atlantic coast, according to the country's electoral commission.
The extended voting was necessary because new voting equipment failed to confirm voters' identities.
Nearly 60 million Nigerians have cards to vote and for the first time there is a possibility that a challenger can defeat a sitting president in the high-stakes contest to govern Africa's richest and most populous nation.
The front-runners among 14 candidates are President Goodluck Jonathan, a 57-year-old Christian from the south, and former military dictator Muhammadu Buhari, 72, from the predominantly Muslim north.
Voters also are electing 360 members to the House of Assembly, where the opposition currently has a slight edge over mr Jonathan's party.
Voting for 13 constituencies was postponed until April because of shortages of ballot papers, electoral officials said.
Nigeria's political landscape was transformed two years ago when the main opposition parties formed a coalition and for the first time united behind one candidate, Mr Buhari. Dozens of parliamentarians defected from Jonathan's party.
A major campaign issue has been Boko Haram's Islamic insurgency. The failure of Mr Jonathan's administration to curb the rebellion, which killed about 10,000 people last year, has angered many Nigerians.
International outrage has grown over the government's inability to rescue 219 schoolgirls kidnapped by Boko Haram nearly a year ago. The extremists have abducted hundreds more people since then, using them as sex slaves and fighters.
The Islamic uprising has exacerbated relations between Christians like Mr Jonathan, who dominate the oil-rich south, and Muslims like Mr Buhari, who are the majority in the agricultural and cattle-herding lands of the north.
Nigeria's population of 170 million is almost evenly divided between Christians and Muslims.
Some 1,000 people were killed in rioting after Mr Buhari lost to Mr Jonathan in the 2011 elections. Thousands of Nigerians and foreign workers have left the country amid fears of post-election violence.
In 2011, there was no doubt that Mr Jonathan had swept the polls by millions of votes. Now the race is much closer. Results are expected 48 hours after voting ends. If no clear winner emerges, a run-off will be held.