Updated at 1:50 p.m. ET

Nigerians turned out in large numbers to select their next president, a contest between incumbent Goodluck Jonathan and his rival, former military dictator Muhammadu Buhari. But the generally peaceful polling was marred by attacks attributed to the Islamist extremist group Boko Haram.

The voting occurred as at least six people were killed in two attacks in the country's northeast, where Boko Haram militants are active. According to Reuters, gunmen opened fire on voters in two locations, killing three at each of them. NPR's Ofeibea Quist-Arcton Ofeibea says two explosions — at least one a car bomb targeted on a polling station — also took place in the country's east, but not in the same area that's known for attacks by Boko Haram militants. There were no immediate reports of deaths, she says.

Despite the violence, Ofeibea reports an "enthusiastic turnout" for what's being described as the tightest race in the history of the West African country, which is taking place amid a deadly Islamist insurgency.

The first step in the process is voter accreditation. Next voters will be allowed to cast ballots. Voice of America says "the process is going slowly in some areas, in part due to delayed voting materials, and in part because biometric card readers are having trouble recognizing voters' fingerprints."

Later, Kayode Idowu, a spokesman for the electoral commission, told NPR that voting would be extended for an extra day at polling stations that had technical problems.

President Jonathan has campaigned on his reputation of being a reliable, albeit cautious hand on the ship of state. Buhari has vowed to get tough on corruption.

According to The Associated Press:

"Polling stations opened late in many areas as official 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."Streets are deserted. Only electoral officials, observers, security officials and media with relevant stickers are allowed to use the roads. All sea and land borders are closed as a security precaution."

Reuters reports that a group calling itself the Nigerian Cyber Army shut down the website of the electoral commission today, warning officials not to rig the election.

"Strucked by Nigerian Cyber Army," read the banner at the top of the page, and underneath: "security is just an illusion."

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