Nigeria's incumbent President Muhammadu Buhari has surged to an early lead in election returns, winning six of 36 states in Africa's largest democracy, while the main opposition rejected the count, alleging manipulation.
Election observers said the last-minute postponement of the vote until Saturday discouraged some Nigerians from going to the polls as Mr Buhari seeks a second term at the head of a country troubled by corruption, insecurity and a weak economy.
Mr Buhari, a former military dictator from the north, faced a strong challenge from opposition candidate Atiku Abubakar, a former vice president and businessman.
The electoral commission announced Mr Buhari had won in six states, with Mr Abubakar taking only the territory that includes the capital, Abuja, and the south-western state of Ondo by Monday night.
The chairman of Mr Abubakar's party, Uche Secondus, accused ruling party agents of hacking into the electoral commission's computer server and manipulating results. He rejected the count as "incorrect, thus unacceptable".
Final results are expected on Tuesday or Wednesday.
African Union observers called the political climate "largely peaceful and conducive for the conducting of credible elections" but urged the country of 190 million people to remain calm.
A Nigerian civil society platform, the Situation Room, cited "major logistic lapses" in the vote. The election was a step back from 2015, praised as one of Nigeria's most transparent and efficient votes, the group said.
At least 39 people were killed in voting-related violence over the weekend. Election observers organised by the Commonwealth, of which Nigeria is a member, called that "deeply troubling", saying Africa's largest democracy can do better.
"The peaceful achievement of millions was overshadowed by the violence of a few," US ambassador Stuart Symington said.
Nigeria's election was originally scheduled for February 16 but the electoral commission postponed it hours before polls were to open, citing logistical issues. The late arrival of election officials and materials, as well as malfunctions and harassment, caused voting to continue in scattered parts of the country into Sunday.
The delays "risk undermining citizen confidence in elections and disenfranchising voters", the US-based International Republican Institute and National Democratic Institute observers said. European Union observers said many people were discouraged from voting.
It was not yet clear how many of Nigeria's estimated 73 million eligible voters turned out. The YIAGA Africa project, which deployed more than 3,900 observers, put it at between 36% and 40%, down from 44% in 2015. It projected that no run-off election will be needed.
Widespread concerns continued about possible incitement to violence by the major political parties, the US-based observers said, while noting that Nigerians have showed resilience and patience so far.
Underscoring those fears, supporters of Mr Buhari and Mr Abubakar briefly confronted each other in the northern city of Kano, pulling knives and machetes.
The overall mood was celebratory, however, as the ruling party anticipated victory in Nigeria's largest city and the heart of the largely Muslim north.