Eighty-nine year old Robert Mugabe won a crushing victory in Zimbabwe's disputed presidential election Saturday, extending his polarising 33-year rule amid claims of widespread vote rigging.

Official results showed Mugabe won 61 percent of the presidential vote and a super-majority in parliament, routing his bitter rival Morgan Tsvangirai who trailed heavily with 34 percent.

Mugabe has ruled Zimbabwe since independence from Britain in 1980, through a series of economic crises, flawed elections and brutal crackdowns that have brought UN sanctions and pariah status.

Western powers voiced serious doubts that the election had been fair but Emmerson Mnangagwa, the defence minister and a key Mugabe lieutenant, argued the result was a game-changer.

"The West will now have to climb down, they must find a ladder and climb down... A democratic election has taken place in Zimbabwe," he told AFP.

Tsvangirai, who has unsuccessfully tried to unseat Mugabe three times, condemned the vote as "fraudulent and stolen."

He vowed to challenge the result in court and said his Movement for Democratic Change would boycott government institutions.

For the last four years Mugabe and Tsvangirai have been involved in an uneasy power-sharing government that helped stabilise the crisis-ridden economy and prevent further bloodshed.

Tsvangirai defended the decision to enter government with the man who has had him arrested, beaten and charged with treason and who has frequently appeared to outflank him in power.

"Our participation rescued this country, schools had closed, hospitals had closed. We were using the Zimbabwe dollar which was worthless, they were no goods in the shops, everyone was desperate," he said.

But furious at the alleged scale of rigging this time round, Tsvangirai said the days of cohabitation were over.

"We will not join government," he said. "We will go to court."

"The fraudulent and stolen election has plunged Zimbabwe into a constitutional, political and economic crisis."

The poll's credibility has been further called into question by the resignation of one of the nine official electoral commissioners.

In a letter seen by AFP Mkhululi Nyathi quit over "the manner" in which the polls "were proclaimed and conducted".

"While throughout the whole process I retained some measure of hope that the integrity of the whole process could be salvaged along the way, this was not to be, hence my considered decision to resign," she said.

The MDC now has until Wednesday to present evidence of fraud to the high court, but finding a smoking gun may prove difficult.

US Secretary of State John Kerry said the results were not "credible" and the electoral process had been "deeply flawed."

"The United States does not believe that the results announced today represent a credible expression of the will of the Zimbabwean people."

British Foreign Secretary William Hague said he had "grave concerns" over the conduct of Wednesday's vote in the former British colony.

The European Union, which had been moving toward easing long-standing sanctions, expressed concern about "incomplete participation, as well as the identified weaknesses in the electoral process and a lack of transparency."

Meanwhile UN chief Ban Ki-moon has urged both political rivals to send "clear messages of calm" to their supporters.

Tsvangirai stopped short of calling his supporters onto the streets, fearing a repeat of the bloody crackdown that followed his win in the first round of 2008 polls.

In Harare late Saturday, there was calm, with little sign of protests or pro-Mugabe victory rallies.

"In 2008 we voted in anger, but this time we knew what we were doing, having experienced the two leaders -- we now know who has the qualities to be a leader," said barber Right Chirombe, 28, said.

Zimbabwe's neighbours have given the vote qualified approval.

The Southern African Development Community, which was engineered the power sharing government, said it was "free and peaceful".

"We did not say it was fair ... we didn't want to jump to a conclusion," said top SADC election observer Bernard Membe.

Tsvangirai said he would submit a dossier of "all irregularities and all the illegalities" to the 15-member bloc and called for an urgent summit.

Even before the official election results, Mugabe followers were planning how to use a parliamentary majority.

"The new constitution will need cleaning up," said Justice Minister Patrick Chinamasa, referring to a text overwhelmingly approved in March that introduced term limits and curbed presidential powers.

Chinamasa said Mugabe's government would also press on with controversial efforts to bring firms under black ownership.

Investors have expressed fears that may mean rolling back the power-sharing government's efforts to stabilise the economy after crippling hyperinflation and joblessness.

"It's back to extreme volatility," Iraj Abedian, the CEO of Pan African Investments, told AFP from Johannesburg.

"We can expect fairly radical positions that will have populist support, but which will have huge implications."