After a number of problems and incidents during election day, voting in parts of Nigeria has been extended until tomorrow (Sunday).

Technical problems have affected voter registration and more than a dozen people are believed to have been killed in various attacks by gunmen. Reuters reports that at least 15 people have been killed, including an opposition politician.

There were security concerns ahead of the election, after the militant group Boko Haram threatened to interfere with polling. However it is unclear whether the group is responsible for all of the fatal attacks on election day.

BBC reported that current president Goodluck Jonathan had to abandon his first attempt to register to vote this morning, with technical problems said to have occurred with a new biometric card system in place for this election. 

The president is facing a strong challenge from ex-military ruler Muhammadu Buhari.

The election is one of the most hotly contested since the country achieved independence, with analysts saying the race is too close to call.

Goodluck Jonathan's People's Democratic Party has been in power since 1999, the year when military rule ended in the country. Buhari is running for the All Progressives Congress party, and this election marks his fourth presidential bid.

There are more than 56 million eligible voters in the country.