Zimbabwe's opposition MDC party on Tuesday claimed victory in the country's historic elections, setting the scene for a showdown with the ruling ZANU-PF that has held power since independence in 1980.
By nightfall, a stream of security forces has locked down the city centre and normally bustling areas like the bus station are deserted, AFP reports.
Opposition supporters have been engaged in running battles with police, with officers using water cannons in an attempt to disperse protestors.
Even before the violence, European Union observers questioned the conduct of the presidential and parliamentary poll, the first since Mugabe's forced resignation after almost 40 years in charge of the Southern African nation.
Supporters of Nelson Chamisa said he was the rightful president of the country following the elections that took place on Monday. "This is not an election it is a disgrace on our country", one young man, Colbert Mugwenhi, said.
The EU observer mission has expressed "serious concerns" as to whether the vote was free and fair - crucial for lifting global sanctions on the once-prosperous country.
The electoral commission had said it would start announcing results for the presidential race from 10:30 GMT, but this was delayed as commissioners read out more parliamentary results. The 94-year-old former leader had been in power since independence from white minority rule in 1980 until he was forced to resign in November after the military and ruling party turned on him. "This time we will not allow it, we will fight", said one protester who wore a red MDC beret in central Harare.
It said the election was largely peaceful in a break from the past but wondered why presidential votes were counted first but were being announced last.
"The strategy is meant to prepare Zimbabwe mentally to accept fake presidential results".
Former President of Zimbabwe Robert Mugabe resigned after 37 years in office
"We won the popular vote & will defend it!"
That would be required if no candidate wins at least 50 percent of the ballots cast by Zimbabwe's 5.6 million voters in the first round.
European Union observers on Wednesday said Zimbabwe's elections had several problems, including media bias, voter intimidation and mistrust in the electoral commission and questioned delays in releasing the presidential results, but the EU does not yet know if the shortcomings in the election will have a material impact on the result.
It called for any aggrieved candidates to "refrain from any form of violence".
"We are working flat-out", commission chairwoman Priscilla Chigumba told journalists in the capital.
Liberia's former President Ellen Johnson Sirleaf, who was monitoring the poll on behalf of the US-based National Democratic Institute, told the BBC that Monday's long queues showed Zimbabweans were enthusiastic about voting, without any kind of repression.
Opposition leader Chamisa accused the ZANU-PF of trying to steal the election.
Mnangagwa was allegedly involved in violence and intimidation during the 2008 elections when then opposition leader Morgan Tsvangirai pulled out of the run-off after attacks claimed the lives of at least 200 of his supporters.