Nigeria's closely fought general election went into a second day on Sunday after failures in controversial new technology snarled the polling, including for President Goodluck Jonathan.
At the same time, military fighter jets and ground troops pounded Boko Haram fighters in the northeastern state of Bauchi after a series of attacks on polling stations on Saturday and Sunday.
The presidential election in Africa's most populous nation 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 former military ruler Muhammadu Buhari.
The prospect of a democratic transfer of power — plus economic woes caused by the slump in global oil prices, concerns about about corruption and fears about insecurity — has energized the vote.
One government spokesman claimed there was a "record turnout" and voting was largely peaceful despite sporadic pockets of unrest mainly in southern states such as the key battleground of Rivers.
The technical difficulties, however, set the tone for a potential dispute as the PDP has opposed the use of the 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 voter fraud that has marred previous elections.
Wrangling over the results has already begun after counting on Saturday, some of it by flashlight with Nigeria regularly plunged into darkness by daily power cuts.
There has been a flurry of claimed constituency successes from both sides, and APC spokesman Lai Mohammed warned about vote manipulation.
"There must be no shenanigans," he said.
The PDP has described the failure of the technology to read biometric data such as fingerprints on the president's own voter identity card as a "huge national embarrassment."
The chairman of the Independent National Electoral Commission (INEC), Attahiru Jega, conceded there were "challenges," as well as the late and even non-arrival of election officials and materials.
But he added: "From our general assessment, out of the 150,000 card readers which we have deployed, only about 450 were affected."
The devices will be used again on Sunday or voters will be processed manually in case of further glitches so that all citizens can exercise their democratic right, he added.
"It will not affect returns on [the] presidential election," he told the private Channels television.
Dozens of polling units were affected in Nigeria's financial hub of Lagos in the southwest.
An electoral officer in the Kosofe local government area told AFP: "We had challenges yesterday with the card readers but it is working fine now."
Voting was also yet to be concluded in nine polling units in the northeastern state of Borno, including four in Maiduguri, where people displaced by Boko Haram violence voted in camps.
All ballots were expected to be cast by Sunday night, said INEC Borno spokesman Tommy Magbuin, with the number of registered voters put at almost 69 million.
A final result had been due within 48 hours of polling stations closing before the extension.
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 militants forcing a six-week delay to the scheduled Feb. 14 election.
On Sunday, residents and a military source said soldiers supported by two fighter jets intercepted the militants at Dungulbe village, seven kilometers (four miles) from Bauchi city in the northeast.
"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 operation is still ongoing but the terrorists have suffered serious losses and are in disarray," the officer added.
The militants were believed to have come through the town of Alkaleri, 60 kilometers away, where there was a dawn raid on Saturday.
Gunmen in several vehicles attacked public buildings, security checkpoints as well as the office of the paramilitaries and the local electoral commission premises.
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 neighboring Gombe state on Saturday killed at least seven.
Boko Haram leader Abubakar Shekau has vowed to disrupt the election, claiming it is "un-Islamic." Polling stations were deemed vulnerable after a spate of suicide and bomb attacks in the north.
(By Ola Awoniyi)