Zimbabwean opposition leader Morgan Tsvangirai has pulled out of a run-off election against President Robert Mugabe.

Mr Tsvangirai said a free and fair poll was impossible in the current climate of violence.

The Movement for Democratic Change (MDC) leader called on the United Nations and the African Union to intervene to stop "genocide" in the former British colony as Western governments condemned Mr Mugabe's "thugs".

The MDC and Mr Tsvangirai, who beat President 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 intimidation and strong-arm tactics to ensure a Mugabe victory in the June 27 poll.

Mr Tsvangirai said: "We in the MDC have resolved that we will no longer participate in this violent, illegitimate sham of an election process.

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

Mr 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.

The MDC earlier said that youths loyal to President Mugabe's Zanu-PF party attacked an MDC rally in Harare, beating journalists and forcing election observers to flee.

"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.

Foreign Secretary David Miliband said: "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.

"He has made, and his thugs have made, an election impossible and so now we face a critical crisis of legitimacy because it's clear that the only people with any shred of legitimacy are the people who won the March 29 first round and that was the opposition."

The White House said in a statement: "The government of Zimbabwe and its thugs must stop the violence now."

South African president Thabo Mbeki said: "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."

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