A European Union observer mission to Mozambique's election raised the alarm on Thursday over unfair conditions and unjustified use of state resources by the ruling party, as well as widespread violence.
Results are not in yet from Tuesday's presidential, legislative and provincial poll. But its outcome will test a fragile peace deal between the ruling Frelimo Party and its old civil war foe turned opposition rival Renamo.
Any reports of irregularities or fraud are likely to put the two-month-old agreement at risk.
"An unlevel playing field was evident throughout the campaign," the EU mission said in a statement.
"The ruling party dominated the campaign in all provinces and benefited from ... incumbency, including unjustified use of state resources, and more police escorts and media coverage than opponents."
Frelimo spokesman Caifadine Manasse said the EU claims were "unfounded".
"Frelimo went to these elections as a governing party, but competed on equal terms with the other parties," Manasse said.
"The European Union is acting badly in making such pronouncements ... We work on the ground using our means, the means of the Frelimo party and the candidate."
The election is widely expected to return President Filipe Nyusi to power, and extend his party's decades-long rule over a southern African nation set to become one of the world's main gas exporters.
Very few results have yet trickled out, although an official at the electoral commission said they may start coming out on Friday - the law allows 15 days in total after the vote.
Under the peace deal, Renamo will be able to choose governors for the first time in any provinces it wins, instead of them being appointed by the central government in Maputo. Yet there are fears of unrest if it fails to win any.
The electoral campaign has been marred by violence.
"Inter-party violence was prevalent as well as mistrust between the main political parties and a lack of confidence that the electoral administration ... [was] independent," the EU said, adding that there was also little public trust in the independence of the police.
The run-up to the vote was marred by sporadic violence, including the killing of an election observer and attacks from a breakaway group of Renamo fighters, with one person reported killed.
African observer missions were more sanguine, with both the African Union team led by former Nigerian President Goodluck Jonathan and the Southern Africa bloc SADC praising the poll for being peaceful and well organised.
"We commend [the electoral commission] and the state for conducting successful, peaceful and orderly elections," SADC mission chief Oppah Muchinguri-Kashiri told journalists.
"Parties and people must be patient and remain committed to Peace as the results are being compiled for validation," said Muchinguri-Kashiri, who is Zimbabwe's defence minister.
The former rebel movement Renamo seeks to control its traditional heartlands in central and northern provinces but it faces the waning popularity of its candidate, Ossufo Momade, and a challenge from the younger opposition party, the MDM.
Momade warned against any manipulation of the results on Tuesday.