President Emmerson Mnangagwa has won Zimbabwe's presidential election with 50.8 per cent of the vote, the country's Electoral Commission says.

Key points: ZANU-PF party also wins parliamentary majority

ZANU-PF party also wins parliamentary majority Opposition rejects results in impromptu televised statement

Opposition rejects results in impromptu televised statement Violence mars poll with six people killed

Officials said Opposition Leader Nelson Chamisa won 44.3 per cent.

Mr Mnangagwa said he was "humbled" by his win in Monday's poll, the first after the fall of his former mentor Robert Mugabe.

"Though we may have been divided at the polls, we are united in our dreams," Mr Mnangagwa said on Twitter.

"This is a new beginning. Let us join hands, in peace, unity & love, & together build a new Zimbabwe for all!" he tweeted, after a week that began with peaceful voting on Monday but spiralled into deadly violence in the capital on Wednesday as the military fired on protesters.

Minutes before Mr Mnangagwa was declared the winner, Morgen Komichi, chairman of Zimbabwe's opposition Movement for Democratic Change (MDC), said the party rejected the results.

Mr Komichi made the impromptu televised statement at the Zimbabwe Electoral Commission, where the results were being announced.

Sorry, this video has expired Zimbabwe opposition figures escorted off stage after rejecting results

"We are not about to be a meal for lions," Mr Chamisa earlier told reporters, alleging Mr Mnangagwa's ruling ZANU-PF party had rigged the poll, but without providing any concrete evidence.

Mr Mnangagwa was a longtime Mugabe confidante before his firing in November led his allies in the military to step in and push Mr Mugabe to resign after 37 years in power.

Since taking office, the 75-year-old Mnangagwa has tried to recast himself as a voice of reform, declaring that Zimbabwe was "open for business" and inviting long-banned Western election observers to observe Monday's vote, which he pledged would be free and fair.

Violence mars election

Sorry, this video has expired Violence breaks out after elections in Zimbabwe (Photo: AP)

The disputed result appears set to deepen a political crisis that was worsened by Wednesday's violence in Harare as the military swept in with gunfire to disperse opposition supporters alleging vote-rigging.

The death toll rose to six, with 14 injured, police said, and 18 people were arrested at the offices of the main opposition party amid tensions over a vote that was supposed to restore trust in Zimbabwe after decades of Mr Mugabe's rule.

Nelson Chamisa's party is refusing to accept the election results. ( AP: Tsvangirayi Mukwazhi )

The violence broke out when the Electoral Commission announced the ruling ZANU-PF party had won a parliamentary majority in the election.

Western election observers who were banned in previous votes have expressed concern at the military's "excessive" force.

Their assessments of the election are crucial to the lifting of international sanctions on a country whose economy collapsed years ago.

Mr Komichi, the MDC party chairman, said the elections were "fraudulent" and "everything has been done illegally".

He said he had refused an Electoral Commission request to sign papers certifying Mr Mnangagwa's win.

"We're not part of it," said Mr Komichi, adding that the opposition would be challenging the election in the courts.

Clashes erupt in downtown Harare on Wednesday. ( AP: Jerome Delay )

Commission chair Priscilla Chigumba urged the country to "move on" with the hopeful spirit of election day and beyond the "blemishes" of Wednesday's chaos.

"May God bless this nation and its people," she said.

With the military still deployed in Harare, the capital's streets were quiet following the announcement of Mr Mnangagwa's victory.

While Mr Mnangagwa and the ruling party accused the Opposition of inciting the violence, the opposition, human rights activists and international election observers condemned the "excessive" force used against protesters and appealed to all sides to exercise restraint.

All is quiet on the streets of Harare on Thursday. ( Reuters: Mike Hutchings )

Police raided the headquarters of Mr Chamisa's party while a lawyers' group said Mr Chamisa was being investigated for allegedly inciting violence.

He and several others are suspected of the crimes of "possession of dangerous weapons" and "public violence", according to a copy of a search warrant.

Mr Chamisa, however, said police seized computers and were looking for what he called evidence that his party had gathered of vote-rigging by Mr Mnangagwa's party.

The evidence already had been moved to a "safe house", he said.

Mr Mnangagwa called for an "independent investigation" into Wednesday's violence, saying those responsible "should be identified and brought to justice".

Reuters/AP