Zimbabwean opposition leader Morgan Tsvangirai pulled out of a run-off election against President Robert Mugabe on Sunday, saying a free and fair poll was impossible in the current climate of violence.

Speaking only hours after his opposition Movement for Democratic Change reported its rally had been broken up by pro-Mugabe youth militia, Tsvangirai called on the United Nations and the African Union to intervene to stop "genocide" in the former British colony.

"We in the MDC have resolved that we will no longer participate in this violent, illegitimate sham of an election process," he told reporters in Harare.

The MDC and Tsvangirai, who beat Mugabe in a March 29 vote but failed to win the absolute majority needed to avoid a second ballot, have repeatedly accused government security forces and militia of strong-arm tactics to ensure a Mugabe victory in the June 27 poll.

Tsvangirai repeated this on Sunday, saying there was a state-sponsored plot to keep the 84-year-old Mugabe in power.

"We in the MDC cannot ask them (the voters) to cast their vote on June 27, when that vote could cost them their lives," he said.

Tsvangirai, who himself had been detained by police five times while campaigning, said 86 MDC supporters had been killed and 200,000 displaced from their homes.

Mugabe has repeatedly vowed never to turn over power to the opposition, which he brands a puppet of Britain and the United States.

Mugabe, who has ruled since independence from Britain in 1980, has blamed the political violence on the opposition and denies security forces have been responsible for brutal actions.

The veteran leader has presided over a ruinous slide in a once prosperous economy. Millions have fled the political and economic crisis to neighbouring states.

A growing chorus of African leaders have added their voices to MDC's concerns that the election would be illegitimate.

Angolan President Jose Eduardo dos Santos, a longtime Mugabe ally, urged him to allow the election to proceed in a spirit of tolerance and with respect to democratic norms.

But one Africa analyst said that despite Tsvangirai's call for intervention from outside there was little prospect of such coming from other African nations.

"It (Tsvangirai's withdrawal) means Robert Mugabe is the legitimate president of Zimbabwe as far as the legal position is concerned," said Tom Cargill, manager of the Africa programme at the London-based Chatham House.

Though Mugabe's air of invincibility had now been destroyed there was little regional countries could do, he said.

The MDC earlier said that thousands of youth militia loyal to Mugabe poured into an MDC rally in Harare on Sunday armed with iron bars and sticks, beating journalists and forcing election observers to flee. But Zimbabwe's government denied this.

"We do not accept that those people were ZANU-PF. We know the MDC has been giving its thugs ZANU-PF regalia to create the impression that we are behind the violence," Justice Minister Patrick Chinamasa said.

Police had banned the rally, which was to be the highlight of Tsvangirai's stormy election campaign, but a high court in Harare overturned the police ban on Saturday.

Tsvangirai said he won the March vote outright and only reluctantly agreed to a run-off. The state-run media has refused to run the opposition's political ads and police have blocked some of its rallies. - Reuters