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Zimbabwe's opposition leader has dismissed "unverified fake results" after President Emmerson Mnangagwa was declared winner of the first election since the ousting of Robert Mugabe.
Nelson Chamisa's MDC Alliance party vowed to launch a legal challenge, saying the vote was rigged.
Mr Mnangagwa has urged Zimbabweans to unite behind his presidency.
Troops 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 took office when 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.
Mr Mnangagwa, of the governing Zanu-PF party, narrowly avoided a run-off by taking 50.8% of the vote, official results show. Mr Chamisa took 44.3%.
The country's 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.
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."
The ZEC responded by saying there was "absolutely no skulduggery".
What has Mnangagwa said?
Mr Mnangagwa, 75, who took over from Mr Mugabe in November, took to Twitter to say he was "humbled" to have won the election.
"Though we may have been divided at the polls, we are united in our dreams. This is a new beginning," he added.
He has called for calm and said the government was in talks with Mr Chamisa to defuse the crisis.
Mr Mnangagwa has proposed an independent investigation to bring to justice those who were behind Wednesday's violence.
"This land is home to all of us and we will sink or swim together," he said in a series of tweets.
What do other countries think?
Neighbouring South Africa's President Cyril Ramaphosa - who is chairman of the Southern African Development Community - has appealed to political leaders and the Zimbabwean people to accept the outcome of the election.
European Union and Commonwealth election observers had earlier criticised the delay.
EU observers also said they had found an "un-level playing field and lack of trust" in the election process.
It was the first time in 16 years that the government had allowed EU, Commonwealth and US election monitors into the country.
Chaos as protesters hit streets of Harare
More on post-Mugabe Zimbabwe:
How have tensions risen?
The day after the election, the MDC Alliance said Mr Chamisa had won the presidential vote, pre-empting an official announcement and prompting its supporters to celebrate in some areas of Harare.
The following day, when Zec announced that Zanu-PF had won the parliamentary vote by a landslide, tensions rose further.
Opposition supporters were also angered by the delay in announcing the presidential results.
There were chaotic scenes in Harare on Wednesday as troops and riot police clashed with MDC Alliance protesters. Six demonstrators were killed and many others injured.