"This time we will not allow it, we will fight", said one protester who wore a red MDC beret in central Harare. "We know that people will be beaten - especially in rural areas, like what they were doing before", said Harare shop worker Tracy Kubara, 26.
At least one person was shot dead by the soldiers near a bus rank, witnesses at the scene told a Reuters photographer.
Police confirmed the death toll of three, and Mnangagwa issued a statement blaming the opposition for the unrest and fatalities.
The EU observer mission expressed "serious concerns" as representatives of Western and other groups gave their first assessments of whether the vote was free and fair - crucial for lifting worldwide sanctions on the once-prosperous country.
That apparently prompted Home Affairs Minister Obert Mpofu to warn that anyone illegally announcing results would "provoke the wrath of the law and risk being sent to jail".
"The opposition. have perhaps interpreted our understanding to be weak, and I think they are testing our resolve and I think they are making a big mistake".
Meanwhile, officials from the electoral commission were read the results from the parliamentary contest in a stuffy meeting room in the Rainbow Hotel.
Although the campaign has been free of the systematic violence that marred previous polls, the MDC has repeatedly claimed it has been hindered by a flawed electoral roll, ballot paper malpractice, voter intimidation and handouts to voters from the ruling party. It called for transparency in the release of results. Many in his party had been on the receiving end of the police beatings and...
In Harare, the contrast could not be stronger with November, when hundreds of thousands filled the streets, hugging soldiers and celebrating their role in ousting 94-year-old Mugabe, the only leader Zimbabwe had known since independence in 1980.
United Nations Secretary-General Antonio Guterres called on Zimbabwe's political leaders and people to exercise restraint and reject any form of violence.
Here’s How Meghan Markle Will Spend Her First Birthday As A Royal
The 36-year-old royal was devastated after her father slammed her in a new newspaper interview at the weekend. What riles me is Meghan's sense of superiority.
With elections under Mugabe marred by fraud and often deadly violence, his successor Mnangagwa, 75, had promised a free and fair vote and invited worldwide observers.
Results so far indicate Zanu-PF won 109 seats, while opposition party MDC claimed just 41, giving incumbent Mnangagwa a term in office.
Rioting erupted on Wednesday (Thursday NZ Time) in Zimbabwe's capital as opposition supporters clashed with police and army troops over delays in announcing results from the presidential election, the country's first since the fall of longtime leader Robert Mugabe.
On Tuesday, Biti alleged at a press conference at MDC headquarters in Harare that the authorities were delaying the publication of results because MDC Alliance leader Nelson Chamisa had won the presidential election.
Mnangagwa also took to Twitter, calling for calm and urging patience before the results were announced.
Their allegation: The commission has delayed its full official announcement in order to rig the vote for incumbent President Emmerson Mnangagwa and his party, the Zimbabwe African National Union-Patriotic Front.
He has already declared he's won the popular vote and the Zimbabwe Electoral Commission is preparing the country to accept "fake presidential results".
His attempts to position Grace as his successor are widely thought to have driven the military to intervene and install their favoured candidate, Mnangagwa, in power.
But Chamisa, a lawyer and pastor, sought to tap into the youth and urban vote.