Africa's oldest leader Robert Mugabe was declared winner of Zimbabwe's disputed election on Saturday, while his main rival Morgan Tsvangirai dismissed the result as a fraud and said he would challenge it in court and in regional forums.

Mugabe, 89, who has ruled the former British colony in southern Africa since its independence in 1980, was formally proclaimed re-elected for a five-year term barely an hour after Tsvangirai announced his planned legal challenge.

"We are going to go to court, we are going to go to the AU (African Union), we are going to go to the SADC (Southern African Development Community)," Tsvangirai angrily told a news conference in Harare. He rejected the result as "fraudulent".

While African observers have already broadly approved Wednesday's peaceful vote, independent domestic monitors have described it as deeply flawed by registration problems that may have disenfranchised up to a million people.

Western observers were kept out by Harare.

In its strongest criticism so far of the poll, the European Union said on Saturday it was concerned about alleged irregularities and a lack of transparency in the election.

The EU's verdict on the fairness of the vote will be crucial to deciding whether it continues to ease sanctions on the southern African country. These were originally imposed because of previous accusations of vote-rigging and abuses of power made against Mugabe and his followers.

Mugabe's ZANU-PF party, which the official results show won a more than two-thirds majority in parliament, has rejected the latest allegations of massive vote-rigging and intimidation made against it by Tsvangirai's Movement for Democratic Change (MDC).

Zimbabwe's Election Commission announced Mugabe had soundly beaten Tsvangirai in the presidential contest with just over 61 percent of the votes, against nearly 34 percent for Tsvangirai.

"Mugabe, Robert Gabriel, of ZANU-PF party, is therefore declared duly elected President of the Republic of Zimbabwe with effect of today," commission head Rita Makarau told a news conference, drawing cheers from ZANU-PF supporters.

Tsvangirai, who had been serving as prime minister in a fractious unity government under Mugabe, said his party would present evidence in court to back its charges that the July 31 vote was a "monumental fraud" engineered by ZANU-PF.

"I thought this election was going to resolve this political crisis. It has not. It has failed. It has plunged the country back to where it was," Tsvangirai said.

He has called on the African Union and SADC to investigate the vote, calling it "null and void" and "not credible."

But he faces an uphill struggle to convince the regional bodies, as their observers have already publicly endorsed the election as free and peaceful, while acknowledging minor problems.

Adding to the controversy surrounding the election, one member of Zimbabwe's nine-member Electoral Commission, Mkhululi Nyathi, has resigned since the vote, citing doubts about the integrity of the results.

Tsvangirai's MDC said on Friday it could take to the streets to challenge ZANU-PF's claim of a landslide victory, made less than 24 hours after the polls had closed on Wednesday.