Zimbabwe's closely-watched elections began with a peaceful vote on Monday, but turned deadly 48 hours later when the military fired on protesters, with six people killed.

On Friday morning three truckloads of police bearing shields and batons tried to disperse about 100 local and international members of the press gathered to hear Movement for Democratic Change (MDC) leader Chamisa speak in a Harare hotel.

With cameras recording their every move, the police eventually pulled back, allowing Mr Chamisa to denounce Mnangagwa's victory.

Chamisa asserted: "We won this election," adding that vote-rigging had given Mr Mnangagwa victory and declared "a day of mourning for democracy".

The MDC leader, who received more than 44 per cent of the vote, alleged violence and harassment against his supporters and manipulation of the election results.

He said his opposition party has evidence of vote-rigging but that the electoral commission "didn't want to listen to us".

There was a subdued reaction to Mnangagwa's win, marked by the apprehension that remained in Harare after the army rolled in with tanks on Wednesday to disperse demonstrators who denounced the president and alleged vote-rigging in the country's first vote following the downfall of long-time leader Robert Mugabe.

The military were not visible on Harare's streets on Friday. Water cannon and police remained present, however, at the headquarters of the main opposition party, a day after authorities raided it and made 18 arrests.

Mnangagwa, Mugabe's former enforcer and confidante, said he was "humbled" by the victory and urged Zimbabweans to stay peaceful.

He said his victory was won fairly and he had nothing to hide, although he criticised the chaotic scenes where police shouting "clear out" chased away journalists waiting for Chamisa's press briefing.

"The scenes today at the Bronte Hotel have no place in our society and we are urgently investigating the matter to understand exactly what happened," Mnangagwa wrote on Twitter.

"We won the election freely and fairly, and have nothing to hide or fear. Anyone is free to address the media at any time."

The opposition said it will challenge in court the results of the election, which Zanu-PF leader Mnangagwa won with just over 50 per cent of the vote.

Chamisa said that "unverified fake results" had been announced by the electoral commission. The commission "must release proper & verified results endorsed by parties," he tweeted.

"The level of opaqueness, truth deficiency, moral decay & values deficit is baffling."

So far, international observers have given a mixed reaction, calling Monday's election peaceful and a break from the past, but expressing grave concern about the military's "excessive" use of force.

They criticised the delay in releasing the results of the presidential vote, saying it raised concerns about possible manipulation.

Australian Associated Press