ABUJA, Nigeria — Election Day violence killed at least 39 people across Nigeria, civil society groups reported, and with ballot-counting temporarily halted and rumors flying, Nigerians were left waiting on Monday for an announcement of the winner in the presidential contest.

With President Muhammadu Buhari vying for a second term against more than 70 challengers, elections officials briefly halted the count, saying that results from Saturday’s voting had yet to be received from all districts.

The 18-hour gap until tabulation began again late Monday morning left time for more rumors to spread about whether the delay was helping the leading candidates rig the results. At one point, members of the party of the leading challenger, Atiku Abubakar, declared victory, a notion that was far from certain.

Much of the violence on Saturday played out in the south of the country, in opposition strongholds where the tight election is particularly contested, according to Cheta Nwanze, head of research at SBM Intelligence, a nonpartisan group that has been critical of the government and is tracking violence based on reports from local police and media.