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Morgan Tsvangirai pulled out of Zimbabwe's presidential run-off election today, complaining that a campaign of violence by Robert Mugabe's supporters had made a fair vote impossible and appealing to the outside world to "stop the genocide".

Mr Tsvangirai announced the decision at a press conference in Harare after a meeting of the national executive of his Movement for Democratic Change (MDC) party.

“We in the MDC cannot ask them to cast their vote on the 27th when that vote would cost them their lives,” he said. “We will no longer participate in the violent illegitimate sham of an election process."

The opposition leader said that President Mugabe had “declared war by saying that the bullet has replaced the ballot" and added: “We believe an election that reflects the will of the people is impossible."

The decision follows a brutal campaign of intimidation by militiamen loyal to Mr Mugabe and his Zanu (PF) party. The MDC says that at last 70 of its supporters have been killed and thousands assault.

Earlier today, around a thousand Zanu (PF) youth militiamen armed with sticks and metal bars invaded a Harare showground where the MDC was to hold a major rally, which had been approved by the High Court despite an attempt by police to ban it.

“Thousands of Zanu (PF) youth militia, armed with iron bars, sticks and other weapons, have attacked journalists and forced election observer teams to flee from the venue of the MDC scheduled rally. Police are firing tear gas,” the MDC said in a statement.

Mr Tsvangirai, who beat Mr Mugabe in a March 29 vote but failed to win the absolute majority needed to avoid a second ballot on June 27, has already been detained by police five times while campaigning. His effective deputy, Tendai Biti, is in custody on a treason charge and other offences that carry a possible death penalty.

Meanwhile, Mr Mugabe, 84, has stepped up his own rhetoric, vowing to cling to power in the country he has ruled since independence from Britain in 1980.

Announcing his withdrawal from the contest, Mr Tsvangirai appealed to the United Nations, African Union and regional body SADC to“intervene and stop the genocide”.

The MDC decision almost certainly hands victory by default to Mr Mugabe, whose aides were already crowing over Mr Tsvangira's decision. “He is aware that he was going to lose dismally, and to avoid a humiliating defeat, he had no option other than to withdraw,” said Patrick Chinamasa, the Justice Minister and a Zanu (PF) spokesman. He added that the election would go ahead unless Mr Tsvangirai formally withdraws in writing.

Mr Mugabe is accused by critics of leading the once model economy to ruin and trampling on human rights. The country has the world’s highest inflation rate and its currency - once at par with the British pound - is in freefall.

A South African mediation team has been in Zimbabwe in an effort to resolve the crisis and South Africa today urged Mr Tsvangirai not to turn his back on negotiations. But the MDC leader said that President Mbeki had made no proposal to him about a national unity government, which has been mooted a possible way out of the crisis.

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