Zimbabwe police have been deployed to the center of the capital ahead of the election results being announced.
Hundreds of people have been singing and dancing in the street outside the MDC party's headquarters since morning.
A large crowd has also blocked the streets in front of the National results center.
Zimbabweans elected their new president and members of parliament on July 31 in the country's first vote since long-time leader Robert Mugabe was forced to step down.
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The presidential ballot included 23 names, with 75-year-old Mnangagwa from the ruling party Zimbabwe African National Union-Patriotic Front (ZANU-PF) and 40-year-old Nelson Chamisa from the MDC party as the main contenders.
Almost 130 parties, including the ZANU-PF and MDC, took part in the parliamentary elections.
Police Disperse Crowd Protesting Against Election Fraud
Soldiers have fired five rounds of ammunition and even used an army helicopter at Zimbabwe election protests.
Armored vehicles arrive in the capital of Zimbabwe, Harare, where people protest after the elections, according to media reports.
The eNCA broadcaster reported that the military has been using live ammunition to disperse the protesters.
According to the state broadcaster ZBC, citing the police, some three people were killed as a result of the clashes.
Tendai Biti of the Zimbabwean opposition Movement for Democratic Change (MDC) party said that security forces on Wednesday have killed two people protesting the election results in Harare.
"We are hearing that six people have been shot and two are dead," Biti told the dpa.
According to the SABC News broadcaster, a large crowd gathered outside the building of an international conference center in Harare to express outrage over the election results, which they consider to be fabricated. The police intervened, dispersing the protesters, the broadcaster said.
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Also on Tuesday, MDC presidential candidate Nelson Chamisa claimed that he won the popular vote, however, the results are yet to be announced. The ruling Zimbabwe African National Union-Patriotic Front (ZANU-PF) party has won the majority of seats in the lower house of parliament.
On July 31, ZEC expressed confidence that there had been no cheating during the presidential election and no complaints from the candidates and political parties had been registered.
Zimbabweans elected a new president and members of parliament on Monday in the country's first vote since long-time leader Robert Mugabe was forced to step down in November 2017. Mnangagwa, the former vice president, was sworn in as Mugabe's successor to serve until the election.