Security is tight outside the offices of the opposition MDC Alliance in Harare. Photo: EPA

Nelson Chamisa says the delay in announcing the results from Monday's election suggests wrongdoing. Photo: AFP

Mnangagwa supporters celebrated after the results were announced. Photo: GETTY IMAGES

Nelson Chamisa's MDC Alliance vowed to launch a legal challenge, saying the vote was rigged. Mr Mnangagwa has urged Zimbabweans to unite behind his presidency.

Police are patrolling the streets of the capital, Harare, after protests on Wednesday left six people dead.

A BBC correspondent in the city centre says a police vehicle with a loudspeaker is broadcasting the message: "Zimbabwe is open for business. We are here to protect you. Feel free to walk and open your business. All is well, fear not."

Mr Mnangagwa, of the governing Zanu-PF party, took office after President Mugabe, 94, was forced to resign in November. The vote was intended to set Zimbabwe on a new path following years of repressive rule, but tensions are now rising.

Electoral Commission

Mr Mnangagwa narrowly avoided a run-off by taking 50.8% of the vote, official results show. Mr Chamisa took 44.3%.

The electoral commission announced the presidential election results from the 10th and final province, Mashonaland West, late on Thursday after days of waiting.

The results of the parliamentary election were announced earlier in the week. They gave Zanu-PF 144 seats; the MDC Alliance, which is made up of seven parties, 64 seats, and one seat to the National Patriotic Front, formed by Mugabe loyalists.

Although Zanu-PF won by a landslide, its majority has shrunk since the 2013 election when it obtained 160 seats and the MDC, then led by the late Morgan Tsvangirai, 49.

What does the opposition say?

"The Zec scandal of releasing unverified fake results is regrettable," Mr Chamisa tweeted on Friday, referring to the Zimbabwe Electoral Commission.

"Zec must release proper & verified results endorsed by parties. The level of opaqueness, truth deficiency, moral decay & values deficit is baffling."

The opposition also questioned the high turnout of more than 80% in most of the country's 10 provinces.

"What they have been trying to do of late is to play around," Mr Chamisa told a news conference hours before the final results were announced.

"That is rigging, that is manipulation, trying to bastardise the result, and that we will not allow."