Tension was high in Nigeria as the country awaited the outcome of the hotly contested presidential poll.
The civil society groups warned that disorganisation and violence may have undermined the polls.
Results from Saturday’s vote were expected to trickle in to Abuja after being collated at the state and local levels.
Mahmood Yakubu, chairman of the INEC election commission, will officially declare the outcome in the coming days. But both sides have already claimed victory.
President Buhari’s aide Bashir Ahmad described the incoming results as “so overwhelming”. “PMB (President Muhammadu Buhari) has been voted by majority and #BuhariIsWinning!” he tweeted.
His main opponent’s media team said INEC should “immediately announce results as delivered from the polling units and declare the people’s candidate, Atiku Abubakar, the winner”.
Mr Yakubu, however, told reporters: “Only the Independent National Electoral Commission can tally figures, announce results and declare winners.”
Whoever becomes the next leader of Africa’s most populous country and leading oil producer faces a daunting to-do list, from tackling widespread insecurity and endemic corruption to boosting an economy recovering from months of recession.
INEC said it was “generally satisfied” with the vote, even though polling stretched into Saturday evening in areas where voter card machines failed and materials were late arriving.
Some affected polling units in six of Nigeria’s 36 states and the Federal Capital Territory of Abuja even saw voting on Sunday.
A coalition of more than 70 civil society groups which are monitoring the vote reported 16 deaths from election-linked violence in eight states. Others gave a higher toll.