Presidential challenger Muhammadu Buhari takes several key northern states amid weekend marred by allegations of fraud.
Results from Nigeria's elections, potentially the closest contest since the end of military rule in 1999, are trickling in after a weekend vote marred by confusion, arguments and sporadic violence.
The electoral commission begain declaring states on Monday with the opposition All Progressives Congress (APC), the party of presidential challenger Muhammadu Buhari, taking the most votes in Oyo, Kogi, Osun and Ondo states.
The ruling PDP took the most votes in Nasawara, Ekiti and Enugu states and the FCT (Federal Capital Territory).
Results from each of the 36 federal states plus the FCT were being tallied at the International Conference Centre in Abuja, the capital.
The Independent National Electoral Commision (INEC) was expected to announce the final results very late on Monday or in the early hours of Tuesday.
On Sunday, protesters in Rivers state, the headquarters of Africa's biggest oil industry, declared the voting as fraudulent.
The United Nations and the African Union, which both had election observers on the ground, said the vote in Africa's most populous nation was broadly in line with "continental and regional principles".
"The AUEOM (African Union Election Observation Mission) encourages all parties to resort to legally established channels, should there be a dispute on the outcome of the process," it said in a statement.
"The AU long-term observers will continue to observe the post-electoral process."
The election pits President Goodluck Jonathan, 57, against 72-year-old former military ruler Muhammadu Buhari for the favour of an electorate divided along a complex mix of ethnic, regional and in some cases religious lines.
Even before preliminary tallies were recorded, the opposition APC rejected the process in Rivers state and denounced the vote there as "a sham and a charade".
"Whatever trash will [be] announced as the result of today's election is not acceptable to us," it said.
The PDP did not respond to requests for comment.
Later, hundreds of sympathisers chanted "APC" outside the electoral commission office in the oil city of Port Harcourt, prompting police to fire warnings shots. One group stoned a car they thought carried ballots.
"There was no election in Rivers," Achinike William-Wobodo, a polling agent for the APC said, calling for a re-vote.
'Appeal to all Nigerians'
In a sign the opposition will challenge results elsewhere, the APC governor of the southern Imo state, Rochas Okorocha, denounced on television the conduct of the election in his region and accused the military of meddling in the result.
Attahiru Jega, INEC chairman, said he was concerned about the Port Harcourt complaints, which alleged that opposition agents were kicked out of vote-tallying meetings, and had launched an investigation.
In Kaduna, the northern city worst-hit by the 2011 post-election violence, the streets were virtually devoid of traffic and many shops were shuttered.
Voting was pushed into an unscheduled second day on Sunday after failures in controversial new technology designed to "read" biometric identity cards to combat electoral fraud.
Among those whose card did not work was the president himself.
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 told a news conference on Sunday.
Deadly attacks
Boko Haram has dominated the campaign, with military operations against the rebels 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 fighters at Dungulbe village, 7km 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 rebels were believed to have come through the town of Alkaleri, 60km 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".
