Nigerians have turned out in their millions to vote in a presidential election that experts say is too close to call between President Goodluck Jonathan and former military dictator Muhammadu Buhari.

Voting was extended yesterday in parts of the country until today after delays and a number of attacks.

Technical problems with new biometric cards slowed down voter registration while car bombs exploded at two polling stations in south-central Enugu state. By yesterday afternoon, suspected gunmen from the Islamist militant group Boko Haram had killed 14 voters, including an opposition politician, in three separate assaults on polling stations in the northeast

In the regional capital Maiduguri, military helicopters hovered above voters waiting in line for an election that has been overshadowed by Boko Haram's bloody six-year campaign to carve out an Islamic caliphate in Africa's most populous nation

Elsewhere the official website of the Independent National Electoral Commission was hacked but was quickly secured, said officials who said the site holds no sensitive material.

Voters in the oil-rich south who traditionally support President Goodluck Jonathan could determine the outcome of the first election in Nigeria's history where an opposition candidate has a realistic chance of defeating a sitting president.

Nigeria's north-east is the centre of the Islamic uprising of Boko Haram who have vowed to disrupt elections, calling democracy a corrupt Western concept.

Thousands of people forced from the homes by the insurgency lined up to vote at a refugee camp in Yola, the north-east Adamawa state capital which is hosting as many refugees as its 300,000 residents.

Polling stations opened late in many areas as officials rushed across the country delivering ballot materials by trucks, speedboats, motorcycles, mules and even camels, in the case of a northern mountaintop village, according to spokesman Kayode Idowu of the Independent National Electoral Commission.

Good humour turned to anger and altercations as people waited hours to be registered to vote, only to find that machines were not reading new biometric voting cards.

Even the president was affected. Three newly imported card readers failed to recognise the fingerprints of Jonathan and his wife. He returned two hours later and was accredited without the machine using visual identification. Biometric cards and readers are being used for the first time to discourage the kind of fraud that has marred previous votes.

Afterward, Jonathan wiped sweat from his brow and urged people to be patient as he had, telling Channels TV: "I appeal to all Nigerians to be patient no matter the pains it takes as long as if, as a nation, we can conduct free and fair elections that the whole world will accept."

Jonathan and Buhari are front-runners among 14 candidates who want to govern Africa's most populous nation.

This is only the eighth election since Nigeria's independence from Britain in 1960. In a country steeped in a history of military coups and bloodshed caused by politics, ethnicity, land disputes, oil theft and, lately, the Boko Haram Islamic uprising, the election is important as Africa's richest nation consolidates its democracy.