Tensions rose after two Mondlane associates were shot dead in Maputo on Saturday.
Lawyer Elvino Dias and Paulo Guambe, a candidate from the small Podemos party which backs Mondlane, were in a car that was surrounded by other vehicles and then they were shot dead, witnesses said.
Podemos leader Albino Forquilha confirmed the killings to AFP, while police said an investigation was launched but did not confirm the identities of the two men.
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The European Union, African Union and United Nations have condemned the incident and called on the authorities to identity the perpetrators.
In a statement, UN Secretary General Antonio Guterres called on “all Mozambicans, including political leaders and their supporters, to remain calm, exercise restraint and reject all forms of violence.”
The head of the African Union Commission, Moussa Faki Mahamat, said he had “deep concern” over “reported cases of post-election violence and in particular the recent killings.”
Maputo was a ghost town on Monday, with shops closed and helicopters hovering above the city of around one million people.
Mozambique, which is awaiting official presidential and parliamentary results this week, has seen past electoral violence. Last year, several people were killed in clashes after municipal elections.