Zimbabwe is on tenterhooks as the country awaits the formal results of the general elections today, amid clashes between the troops and opposition protesters in the capital Harare.

Harare turned bloody yesterday as three people were killed during demonstrations by the opposition over alleged vote fraud, with President Emmerson Mnangagwa calling for calm.

The police confirmed the deaths, adding that the deceased were yet to be identified.

But the deployment of soldiers and their beating up of unarmed protesters is a setback to Mnangagwa's efforts to shed Zimbabwe's pariah status after decades of repression under former president Robert Mugabe, who was ousted in a coup in November.

Gunfire crackled in the streets while troops, some with their faces masked, backed by armoured vehicles and a military helicopter, cleared the streets.

The unrest started soon after opposition Movement for Democratic Change (MDC) leader Nelson Chamisa claimed he had won the popular vote.

Even before the violence, European Union observers questioned the conduct of the presidential and parliamentary poll, the first since Mugabe's forced resignation after nearly 40 years in charge of the Southern African nation.

Scores of Chamisa's supporters who had been burning tyres in the streets then clashed with riot police near the Zimbabwe Election Commission (ZEC) headquarters. Officers responded with tear gas and water cannon.

"I was making a peaceful protest. I was beaten by soldiers," said Norest Kemvo, who had gashes to his face and right hand.

"This is our government. This is exactly why we wanted change, but they are stealing our vote," said Kemvo with bloodied lips.

United Nations secretary-general Antonio Guterres called on Zimbabwe's political leaders and the general public to exercise restraint and reject any form of violence ahead of the announcement of the results today.

The EU observers expressed concern about delays in releasing the results of the presidential contest - a two-horse race between Chamisa and Mnangagwa.

The electoral commission had said it would start announcing results for the presidential race from 12.30pm but that was then pushed back by at least 24 hours.

With three seats yet to be declared in the parliamentary contest, Zanu-PF had 144 seats compared to 61 for the MDC, meaning the ruling party achieved the two-thirds majority that permits it to change the constitution at will.

Chamisa said the early release of the parliamentary results was a deliberate ploy to prepare Zimbabweans for a victory by Mnangagwa, a former national security chief nicknamed 'The Crocodile' and commonly known as ED.

"The strategy is meant to prepare Zimbabweans mentally to accept fake presidential results. We've more votes than ED. We won the popular vote [and] will defend it," Chamisa said on Twitter.

Before the violence, EU chief observer Elmar Brok said he did not yet know if the shortcomings would have a material effect on the outcome of the vote, but criticised the electoral commission for being at times "one-sided".

"The longer it lasts that the presidential result is not known, the more lack of credibility it provides," said Brok, who also called for calm.

Zimbabwe was once one of Africa's most promising economies but slowly became a shadow of its former self as it descended into corruption, mismanagement and diplomatic isolation in the latter stages of Mugabe's administration.

Its population of about 13million is struggling amid shortages of foreign currency, and unemployment above 80% due to lack of foreign investment.