HARARE (Reuters) - 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.

There was no immediate reaction from Mugabe who in the past has blamed election violence on the opposition.

But his justice minister, Patrick Chinamasa, said Zimbabwe would proceed with next Friday's poll unless Tsvangirai officially notified the election authorities he was pulling out.

In a later statement, the MDC said army helicopters were patrolling the skies over Harare and Bulawayo, the second largest city, and that Zimbabwe was effectively under military rule.

U.S. CONDEMNS GOVERNMENT "THUGS"

In Washington, a White House spokesman said: "The government of Zimbabwe and its thugs must stop the violence now."

"All parties should be able to participate in a legitimate election and not be subject to the intimidation and unlawful actions of the government, armed militias and so-called war veterans," said Carlton Carroll, a White House assistant press secretary said in a statement.

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 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 in the past denied that his security forces have been responsible for brutal actions.

If Tsvangirai does formally pull out, Mugabe would then be sworn in for another five-year term. But he could face difficulties governing because the MDC won control of the parliament in a March election.

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.

South Africa said it believed the door was not completely closed to some form of mediation between the two men.

"We are very encouraged that Mr. Tsvangirai, himself, says he is not closing the door completely on negotiations," said Mukoni Ratshitanga, spokesman for South African President Thabo Mbeki, who is mandated by regional bloc SADC to mediate between the opposition and the ZANU-PF.

Little chance of intervention
A growing chorus of African leaders have added their voices to MDC's concerns that the election would be illegitimate and are unlikely to be as accommodating to Mugabe in future as they were in the past.

But one Africa analyst said that despite Tsvangirai's call for intervention 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.

"What Mugabe has stressed since the year dot is sovereignty. Part of that is directed against Western colonial interests, but it can be as effectively directed against regional leaders."

The MDC earlier said that thousands of youth militia loyal to Mugabe on Sunday poured into an MDC rally in Harare that was to have been a high point of Tsvangira's campaign. Armed with iron bars and sticks, they beat journalists and forced election observers to flee, the MDC said.

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 Chinamasa said.