Zimbabwe's 89-year-old Robert Mugabe romped to victory in presidential and parliamentary polls on Saturday, but his longtime foe Morgan Tsvangirai vowed to boycott the government formed by the "fraudulent" vote.

The veteran leader scored another five years in office, extending his 33-year rule with a landslide 61 percent of the vote, against Tsvangirai's 34 percent.

In parliament, his ZANU-PF party scored a super majority which allows it to make changes to the country's constitution.

The result came as a massive blow to longtime opponent Tsvangirai, who said his Movement for Democratic Change "totally" rejected Wednesday's vote and would boycott the incoming government.

The election ends an uneasy power-sharing government with Mugabe installed in 2009 after another disputed vote.

Prime Minister Tsvangirai swiftly ruled out joining Mugabe's government again.

"We will not join government," he said.

"We will go to court," he insisted, after his party held emergency talks to plot its next move.

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

ZANU-PF had already claimed victory on Friday.

Party spokesman Rugare Gumbo told AFP: "Our opponents don't know what hit them."

The MDC rejected the results but has stopped short of calling for mass protests, as growing tensions spark fears of a repeat of the bloody violence that marked the aftermath of the 2008 election.

Amid observer concerns over the electoral roll and high numbers of voters being turned away, the poll's credibility was hit by the resignation of one of the nine official electoral commissioners.

In a letter seen by AFP that was sent to Mugabe and Tsvangirai, dated the day of the polls, Mkhululi Nyathi said he had 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," he said.

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

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

Harare barber Right Chirombe said there was no basis to the claims that the poll was rigged.

"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," Chirombe, 28, said.

The influential 15-member Southern African Development Community has also implored "all Zimbabweans to exercise restraint, patience and calm".

The bloc stopped short of declaring the vote "fair" but 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, while adding that he would try to convince Tsvangirai to concede defeat.

SADC negotiated the creation of a power-sharing government in the wake of 2008's bloody poll.

With 600 observers on the ground, SADC's verdict and next steps will be closely watched by Western nations barred from monitoring the poll themselves.

However, foreign diplomats have privately described the polls as fundamentally flawed, and the independent Zimbabwe Election Support Network reported up to one million voters were prevented from voting in Tsvangirai strongholds.

Even before the official election results, Mugabe followers were already 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."

Mugabe -- Africa's oldest ruler -- is a former guerrilla leader who guided Zimbabwe to independence in 1980 from Britain and white minority rule.

But his more than three-decade military-backed rule has been marked by economic meltdown and international diplomatic isolation.

Former union boss Tsvangirai won the first round of voting in 2008, but was forced out of the race after 200 of his supporters were killed and thousands more injured in suspected state-backed intimidation and attacks.

burs-jg/arb/fb