|PIC1|Zimbabwean opposition leader Morgan Tsvangirai pulled out of a run-off election against President Robert Mugabe on Sunday, saying his supporters would be risking their lives if they voted.

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.

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.

Tsvangirai said there was a state-sponsored plot to keep 84-year-old Mugabe in power. Mugabe has ruled since independence from Britain in 1980.

"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 has blamed election violence on the opposition.

But election authorities said Zimbabwe would proceed with Friday's poll because Tsvangirai had not officially notified them he was pulling out.

The MDC said it would send a letter to the electoral commission confirming Tsvangirai was withdrawing.

The opposition party said army helicopters were patrolling over Harare and Bulawayo, the second-largest city, and that Zimbabwe was effectively under military rule.

More than 2,000 youth members of Mugabe's ruling ZANU-PAF party were on the rampage, attacking citizens in central Harare, the MDC said.

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," Carlton Carroll, a White House assistant press secretary, said in a statement.

U.N. Secretary-General Ban Ki-Moon called the situation in Zimbabwe "deeply distressing."

"The campaign of violence and intimidation that has marred this election has done a great disservice to the people of the country and must end immediately," he said in a statement.

The United Nations is prepared to work urgently with Southern African Development Community and the African Union to help resolve the impasse, Ban said.

Thabo Mbeki, president of leading regional power South Africa, said he would encourage Mugabe and Tsvangirai to discuss the political crisis.

"From our point of view it is still necessary that the political leadership of Zimbabwe should get together and find a solution to the challenges that face Zimbabwe," said Mbeki who is mandated by regional bloc SADC to mediate between the opposition and the ZANU-PF.

Zambian President Levy Mwanawasa, the current SADC chairman, said the run-off should be postponed "to avert a catastrophe in this region," a view echoed by Ban.

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

Once Tsvangirai pull outs, 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.

Former colonial power Britain said the people had deserted Mugabe.

"We have reached an absolutely critical moment in the drive by the people of Zimbabwe to rid themselves of the tyrannical rule of Robert Mugabe," Foreign Secretary David Miliband said.

The MDC earlier said that thousands of youth militia loyal to Mugabe poured into an MDC rally in Harare. 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 Patrick Chinamasa said.