Mozambique's main opposition party Renamo on Saturday called for this week's election to be annulled, accusing the government of breaching a peace deal by using violence and intimidation on voting day.
Partial results from Tuesday's election show President Filipe Nyusi and his ruling Frelimo party headed for a major victory but the poll was marred by claims of fraud and irregularities.
Renamo accused Frelimo of violating a "cessation of hostilities agreement" under the August peace deal which states no party should "commit acts of violence and intimidation in the pursuit of political objectives".
The party said some of its delegates were arrested after they caught poll organisers handing out more than one Frelimo ballot paper to voters.
"There were arbitrary arrests of agents... and voters who tried to complain about anything, ballot box stuffing, being unable to exercise the right to vote," Renamo said in a statement.
It called for a cancellation of the vote and asked for "new elections that must be supervised by reputable entities".
Government officials were not immediately available for comment.
The vote was seen as a test of the fragile peace deal between Frelimo and Renamo, a former rebel group.
The two sides fought a brutal civil war from 1975-1992, devastating the economy and leaving almost one million people dead.
They picked up arms again from 2013 to 2016, although tensions continued until the peace deal was sealed.
'Lack of rigour'
As part of the accord, Frelimo had also agreed to allow voters to elect governors for the country's 10 provinces.
Renamo had been tipped to win control of at least three of those provinces, but partial results have cast doubt on that prediction.
While most international observers agreed the vote was relatively well conducted, the opposition has decried fraud since the start of campaigning.
Civil society groups have said they found evidence of suspected election rigging, including several attempts to stuff ballots and election observers being kicked out of polling stations.
Before the vote, the groups estimated there were hundreds of thousands of "ghost voters" -- names not aligned with real voters -- on the electoral roll.
The US embassy in Mozambique's capital Maputo echoed their claims by noting a "lack of rigour" in the vote counting process.
The National Electoral Commission (CNE) is expected to announce the result of the provincial elections on Monday, and final results within 15 days of the vote.
Frelimo -- which has ruled the southern African nation since independence from Portugal in 1975 -- is widely expected to win the presidential, parliamentary and provincial polls.