Voting in Nigeria's presidential elections was extended one day, until Sunday, in several areas because ballot paper arrived late or new digital voting card readers failed, officials said Saturday, reports CNN.

Gunmen killed at least 15 people including an opposition politician near polling stations in northeast Nigeria on Saturday, casting an ominous shadow over the closest electoral contest since the end of military rule in 1999, reports Reuters.

The tense race pits President Goodluck Jonathan against former military ruler Muhammadu Buhari for the favor of an electorate divided along a complex mix of ethnic, regional and in some cases religious lines.

The poll is seen as the first election in Africa's most populous nation in which an opposition candidate has a serious chance of unseating the incumbent, and widespread fears it could trigger violence are already becoming reality.

Islamist Boko Haram insurgents launched several attacks on voters in the northeast, killing three in Yobe state and three more in Gombe state, police said.

Shortly afterwards, at least eight people, including the opposition parliamentary candidate for Dukku in Gombe, were killed by unidentified gunmen, a spokesman for Buhari's All Progressives Congress (APC) said.

The militants, who are trying to revive a medieval Islamic caliphate in religiously mixed Nigeria, reject democracy and their leader Abubakar Shekau has threatened to kill those who go to vote.

A string of military victories by troops from Nigeria and neighboring Chad, Cameroon and Niger has reclaimed much of the territory the Islamists controlled earlier this year, but they retain the ability to mount deadly attacks on civilians.

The governor of Borno state in the northeast said 25 people had been killed in an assault on the remote village of Buratai on Friday night.

Shortly afterwards, at least eight people, including the opposition parliamentary candidate for Dukku in Gombe, were killed by unidentified gunmen, a spokesman for Buhari's All Progressives Congress (APC) said.

The militants, who are trying to revive a medieval Islamic caliphate in religiously mixed Nigeria, reject democracy and their leader Abubakar Shekau has threatened to kill those who go to vote.

A string of military victories by troops from Nigeria and neighboring Chad, Cameroon and Niger has reclaimed much of the territory the Islamists controlled earlier this year, but they retain the ability to mount deadly attacks on civilians.

The governor of Borno state in the northeast said 25 people had been killed in an assault on the remote village of Buratai on Friday night.