ABUJA (AFP) - Nigerians on Monday awaited the first results of a closely fought general election pitting President Goodluck Jonathan against ex-military ruler Muhammadu Buhari, as UN chief Ban Ki Moon called for calm after deadly riots followed 2011 polls.
The United Nations Secretary General congratulated Nigeria for holding a "largely peaceful and orderly" ballot, as the head of the country's electoral commission said Sunday that the first results could be announced the following day.
But Ban called on citizens to "maintain a peaceful atmosphere and to exercise patience" and condemned attacks carried out by Boko Haram and other militants attempting to disrupt the presidential and parliamentary polls.
Military fighter jets and ground troops pounded Boko Haram fighters in the northeastern state of Bauchi on Sunday after a series of attacks on polling stations at the weekend.
The presidential election in Africa's most populous nation and largest oil producer is the closest in the country's history, with the first credible challenge from an opposition party.
Jonathan's Peoples Democratic Party (PDP) has been in power since Nigeria returned to civilian rule in 1999 but is being pushed to the wire by main opposition candidate Buhari.
The prospect of a democratic transfer of power - plus economic woes caused by the slump in global oil prices, concerns about corruption and fears about insecurity - served to energise the vote.
Voting was pushed into an unscheduled second day Sunday after failures in controversial new technology but election chief Attahiru Jega said his commission was confident its objective of holding a "free, fair, credible and peaceful" election was "on course".
"We appeal to all Nigerians to remain peaceful as they await the return of these results," he added, amid fears of a repeat of 2011 post-poll violence that left some 1,000 people dead.
One government spokesman claimed there was a "record turnout" and that voting was largely peaceful despite pockets of unrest mainly in southern states such as the key battleground of Rivers.
The technical difficulties of the voting process, however, set the tone for a potential dispute as the PDP has opposed the use of handheld electronic devices to authenticate voters, saying they were not sufficiently tested.
Buhari's All Progressives Congress (APC) supports the new system as a means of curbing the voter fraud that has marred previous elections.
Wrangling over the results began as the counting got under way, some of it by flashlight in a country which is regularly plunged into darkness by daily power cuts.
In oil-rich Rivers state, thousands of opposition supporters demonstrated to call for the cancellation of the elections locally because of alleged irregularities.
The ruling PDP earlier described the failure of the technology to read biometric data such as fingerprints - including on the president's own voter identity card - as a "national embarrassment".
The technical glitches, along with the late and even non-arrival of election officials and materials in some areas, led Jega to concede there were "challenges".
But the electoral chief stressed that only a small percentage of the card readers experienced problems.
The devices were used again on Sunday but voters could also be processed manually if problems occurred.
All ballots were expected to be cast by Sunday night, with nearly 69 million people registered to vote.
To avoid a run-off, presidential candidates need to have won the most votes and at least 25 percent support in two-thirds of Nigeria's 36 states and the Federal Capital Territory of Abuja.
Boko Haram has dominated the campaign, with military operations against the Islamists forcing a six-week delay to the scheduled February 14 election.
On Sunday, residents and a military source said soldiers supported by two fighter jets intercepted the militants at Dungulbe village, seven kilometres (four miles) from Bauchi city in the northeast.
A spokesman for the Bauchi state governor said an indefinite, round-the-clock curfew had been imposed on three areas because of the fighting.
The militants were believed to have come through the town of Alkaleri, 60 kilometres away, where there was a dawn raid on Saturday.
Bauchi police spokesman Haruna Mohammed confirmed that polling stations in nearby Kirfi were attacked on Sunday and election materials were destroyed.
A series of suspected attacks on polling stations in neighbouring Gombe state on Saturday killed at least seven.
Boko Haram leader Abubakar Shekau had vowed to disrupt the election, calling it "un-Islamic".