ABUJA: Confusion and violence blighted Nigeria’s tensest presidential election since the end of army rule, with the opposition rejecting results from a turbulent southern state even before they have been announced.

The opposition All Progressives Congress (APC) in Rivers state accused supporters of President Goodluck Jonathan of being behind killings of its campaigners, and denounced the vote there as “a sham and a charade.”

Dismissal of the vote in Rivers — a center of the Nigerian oil industry — raises the prospect of a disputed national outcome and the risk of a repeat of violence that erupted after the last election in 2011, when 800 people were killed in the mainly Muslim north.

Voting in Saturday’s election was extended into Sunday at a relatively small number of polling stations after technical glitches hit voter ID machines.

Witnesses said Boko Haram militants killed more than a dozen voters, while at least two people were shot dead in the oil hub of Port Harcourt, the biggest city in Rivers state which has a long history of political thuggery.

Nigerian troops battled scores of Boko Haram fighters outside the northeastern city of Bauchi Sunday, residents and a military source told AFP.

Soldiers supported by two fighter jets intercepted the militants who were in 20 pick-up trucks at Dungulbe village, 7 km from the city, and engaged them in fierce fighting.

“The fighter jets are pounding the enemy position while ground troops are engaging them,” said a military officer in the city, who asked not to be identified, in an account supported by residents.

The vote pits 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 in Africa’s most populous nation.

It is also the first election since the end of military rule in 1999 in which an opposition candidate has a serious chance of unseating the incumbent.