President Emmerson Mnangagwa has called for Zimbabwe to unite behind him after he was declared winner of national elections, but the opposition leader insisted he had won and said he would use all means necessary to challenge the result.

Mr Mnangagwa said his victory was won fairly and he had nothing to hide, although he criticised chaotic scenes where police shouting "clear out" chased away journalists waiting for a briefing by his main rival Nelson Chamisa.

Mr Chamisa later told reporters that Mr Mnangagwa's ruling Zanu-PF had used deadly violence against opposition supporters following the vote because it had lost the election - the first since the army removed 94-year-old Robert Mugabe from office in November.

"We are going to explore all necessary means, legal and constitutional, to ensure that the will of the people is protected," Mr Chamisa said.

Voting passed off relatively smoothly on the day, raising hopes of a break from a history of disputed and violent polls.

But an army crackdown on opposition supporters in which six people were killed and opposition claims that the vote was rigged revealed the deep rifts in Zimbabwean society that developed during Mr Mugabe's four decades in power, when the security forces became a byword for heavy-handedness.

After three days of claims and counter claims, Mr Mnangagwa - a 75-year-old former spy chief under Mr Mugabe - secured victory.

He polled 2.46 million votes against 2.15 million for 40-year-old opposition leader Mr Chamisa, the Zimbabwe Electoral Commission (ZEC) announced in the early hours of this morning.

Emmerson Mnangagwa said he won the election 'freely and fairly'

"This is a new beginning. Let us join hands, in peace, unity and love, and together build a new Zimbabwe for all," Mr Mnangagwa said on Twitter.

Later, after the disturbance ahead of Mr Chamisa's news conference, he added: "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," Mr Mnangagwa wrote on Twitter.

"We won the election freely and fairly, and have nothing to hide or fear. Anyone is free to address the media at any time."

But his efforts to rehabilitate the image of a country synonymous with political repression and economic collapse were undermined by a police raid on the headquarters of Mr Chamisa's Movement for Democratic Change (MDC) and the dispersal of journalists by riot police at Mr Chamisa's news conference.

Mr Mnangagwa received 50.8% of the vote, just edging over the 50% mark needed to avoid a runoff.

The delays in announcing the presidential results and the narrow margin of victory fuelled the opposition accusations of rigging.

He now faces the challenge of persuading the international community that the army crackdown and lapses in the election process will not derail his promise of political and economic reforms needed to fix a moribund economy.

European Union observers said on Wednesday the elections had several problems, including media bias, voter intimidation, and mistrust in the electoral commission.

Its final assessment will be critical in determining whether Zimbabwe can return into the international fold. Mr Chamisa, who earlier accused the election commission of trying to rig the vote, said that it should release "properand verified" results.

He told a news conference he would pursue all legal means necessary to challenge the result, which had serious legitimacy problems.

He declined to divulge the specific action that his party would take to challenge the outcome of the election.