Mozambique's main opposition party has called for Tuesday's elections to be cancelled, citing violence and fraud.
Renamo accused the ruling party, Frelimo, of violating the August peace deal between the two sides.
A European Union observer mission said the elections were marred by an uneven playing field, violence and a climate of fear.
A Frelimo spokesman dismissed the EU statement as "unfounded", saying the elections passed off smoothly.
The election is seen as a test of the peace deal between the two parties, which fought a bitter 1975-1991 civil war, with renewed hostilities from 2013-16.
With preliminary results in from about a third of polling stations, President Filipe Nyusi has a commanding lead in the presidential race, with about 70% of the votes. Renamo's Ossufo Momade is trailing on about 25%.
Renamo general secretary André Magibiri on Saturday rejected the results being announced, saying they did not correspond to the will of the electorate.
He said the electoral process was "brutal and barbaric, with total violence, arbitrary arrests, ballot-box stuffing, and other irregularities".
Election commission officials have denied reports of ballot-stuffing on election day.
About 10 people have been killed in election violence, according to a local observer mission.
The latest deaths were a Renamo official whose body was found with multiple gunshots, along with that of her husband after they went missing on election day, according to Human Rights Watch researcher Zenaida Machado.
Five days before the election, an election observer was shot dead allegedly by a group of elite police officers.
The police say they are investigating the circumstances in which Anastacio Matavel was shot dead.