As many as 39 people have been killed following Saturday's presidential election, civil society groups have said. Electoral authorities are expected to announce the winner of the vote on Monday.
Unrest and uncertainty has gripped Nigeria after a second day of vote counting following the presidential election. Human rights groups have reported the deaths of 39 people, while Nigerian police have confirmed some 128 arrests following election-related violence.
Saturday's presidential election was fraught with difficulty, having been delayed a week by the electoral commission at the last minute, citing logistical challenges.
Read more: Vote counting begins in Nigeria's delayed presidential election
President Muhammadu Buhari, 76, a former military ruler, is seeking a second term and has been challenged by Atiku Abubakar, 72, a businessman and former vice president.
Official results are expected as early as Monday, though some voting took place on Sunday in scattered areas. Both Buhari and Abubakar have claimed victory in the vote.
The Situation Room, a group that incorporates more than 70 civil society organizations, said 39 people have died since the poll took place, citing information from Lagos-based consultancy SBM Intelligence.
Authorities have not confirmed the figures, but were bracing for the possibility of continued unrest once results are announced this week.
The worst incidence of violence took place in the town of Abonnema, in Rivers State, some 14 kilometers (9 miles) west of the main oil industry city of Port Harcourt, the Situation Room said.
Seven people died there in a shootout between an unidentified gang and Nigerian soldiers, army sources said. The firefight began when attackers barricaded a major road into the town and ambushed the troops, Nigeria's army said.
'Cache of explosives' found
Nigerian police said 128 people have been arrested for suspected election-related offenses. The crimes included ballot box-snatching, vote-trading and impersonation.
Additionally, authorities said they found a "cache of explosives" that included 38 "assorted weapons." Police did not release further details on the arms seizure.
Nigeria is Africa's most populous country and the continent's largest economy. Some 72 million people were eligible to vote on Saturday.
The Situation Room were critical in their assessment of the elections, calling it a step back from Nigeria's most transparent and efficient election in 2015.
The civil society platform cited "major logistic lapses" including widespread delays and "disappointing" conduct by political parties.
However, the African Union observer mission said Monday that the elections "overall political climate remained largely peaceful and conducive for the conducting of credible elections."
"The 2019 elections took place in a generally peaceful environment," said Hailemariam Desalegn, head of the AU observer mission. The union did, however, criticize poor preparation and called for calm until the final results.
jcg,dv/ng (AP, Reuters, AFP)