The results come after opposition leader Morgan Tsvangirai declared the process “null and void”, while the two main observer groups said voting had been “very free” and “very peaceful”.
Most monitors nonetheless accept that there are “serious” concerns around how the electoral registers were drawn up, and a spokesman for the defeated opposition MDC party said “Demonstrations and mass action are options.”
Earlier on Thursday a senior aid to Mugabe told Reuters the party of the 89-year-old incumbent was already confident he would extend his 33 year rule.
“We’ve taken this election. We’ve buried the MDC. We never had any doubt that we were going to win,” he told the news agency.
ZANU-PF’s announcement was followed by Prime Minister Morgan Tsvangirai effectively conceding defeat, while at the same time discounting the process.
Mr Tsvangirai, who leads the Movement for Democratic Change (MDC), called the election a “monumental fraud”.
“It is our view that this election is null and void and does not meet international standards for a credible, legitimate, free and fair election. Once again Zimbabweans have been short-changed.
“We want to assure every Zimbabwean that this is the beginning of the end. Zanu-PF may have this victory as they would like to claim, but I would like to assure you that the resolution of the Zimbabwe crisis has never been so near.”
Having slammed the “shoddy manner” with which polls were conducted, MDC released a statement saying it received “radical” calls to take violent action from its supporters, reflecting “the frustration, hopelessness and anger in every one of us”.
“We understand the tears in everyone’s eyes but we certainly do not believe in violence to resolve this,” it said.
Rugare Gumbo, a spokesman for Mr Mugabe’s ZANU-PF party, categorically denied all accusations of vote-rigging.
“We dismiss these allegations with the contempt they deserve because there was absolutely no way of manipulating the system. There was no way of rigging the election,” he said.
While the two main parties continued to disagree over the legitimacy of the elections, the main observing groups appeared to reach a consensus that they were indeed “fair”.
Mr Mugabe’s government banned Western missions from monitoring the process, leaving a great deal of responsibility for an international verdict in the hands of the African Union (AU) and the Southern African Development Community (SADC).
The leader of AU’s 70-member observer mission Olusegun Obasanjo said: “Yes, the election is free.”
He described the vote as credible unless any evidence to the contrary emerges, though observers admitted they had seen “incidences that could have been avoided and even tended to have breached the law”, and the group has asked election authorities to investigate.
South African agency Eyewitness News reported that the SADC said yesterday a group of their observers had seen “incorrect papers” put in ballot boxes.
In spite of this, they released a verdict today that said the election had been both “very free” and “very peaceful”.
Solomon Zwana, head of domestic organisation the Zimbabwe Election Support Network, was the only major independent group to deliver a damning verdict.
It found a “wide range of problems” in the election and that it had witnessed a campaign to stop voters from casting ballots.
The monitoring group said that the names of as many as 1 million out of more than 6 million eligible voters had not been added to polling booth registers.
The atmosphere in Harare was subdued following the announcement of the two-thirds victory for Mr Mugabe’s party, in stark contrast to the deadly violence that erupted after a disputed presidential poll in 2008.
And a spokesman for the EU said its foreign policy chief, Baroness Catherine Ashton, had “commended the people of Zimbabwe for turning out in large numbers to cast their votes, in what were broadly peaceful elections.”
He said: “She calls upon on all parties to maintain calm and order as the results of the poll become known.”