Emmerson Mnangagwa declared the result to be a "new beginning" for the country
The Zimbabwe opposition leader has vowed to challenge the election result "legally and constitutionally", after saying it was rigged.
The electoral commission announced that President Emmerson Mnangagwa, a former enforcer of long-time leader Robert Mugabe, and leader of the Zanu-PF Party, had narrowly won Monday's vote.
Nelson Chamisa, leader of the Movement for Democratic Change (MDC) Party, said the election had "serious credibility and legitimacy issues" and that he was declaring "day of mourning for democracy".
He said: "I hope Emmerson Mnangagwa is open for business - the business of transparency."
Mr Chamisa, who received 44% of the vote, said the opposition had evidence of vote-rigging but the electoral commission "didn't want to listen to us".
He declared that "we won this election" and said Mr Mnangagwa should acknowledge that.
Mr Chamisa said: "We are not accepting this fiction. We want a proper result to be announced.
"We will pursue all means necessary, legal, constitutional, to make sure we protect the people's vote."
Earlier, riot police broke up the news conference and chased away reporters without explanation, but the event was later allowed to go ahead.
Mr Mnangagwa condemned the violence, saying he is "are urgently investigating" what happened.
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. Over the past nine months we have protected freedom of speech, of assembly and the right to criticise the government. (1/2)— President of Zimbabwe (@edmnangagwa) August 3, 2018
With all 10 provinces declared, the Zanu-PF leader gained 2.46 million votes (50.8%) against 2.15 million (44.3%) for the opposition leader.
The election is the first since dictator Robert Mugabe resigned last year, ending 30 years of leadership.
Violence surrounding the election has seen the military sweep in and use live rounds to disperse protests over alleged vote rigging.
Elections meant to restore trust in Zimbabwe after decades of Mugabe rule have instead seen familiar scenes of violence and claims of vote rigging.
There were conflicting accounts as to who was responsible for the bloodshed in the capital Harare, which followed opposition protests after it was announced Zanu-PF had won a majority in parliament.
Mr Mnangagwa blamed the MDC, saying "it is not entirely true protesters were not armed".
Mr Chamisa, a lawyer and pastor, blamed Zimbabwe's "violent government", saying: "We have unarmed civilians being attacked. Is that normal even in a banana republic?"
International election observers earlier called for the result of the presidential vote to be released as soon as possible to ease tensions, saying delays will add to any speculation the result was manipulated
A credible vote is seen as paramount if international sanctions imposed during the Mugabe era are to be lifted, giving Zimbabwe's moribund economy the chance to recover in the process.