Zimbabwean opposition leader Morgan Tsvangirai has pulled out of a run-off election against President Robert Mugabe.Speaking after his opposition Movement for Democratic Change reported its rally had been broken up by pro-Mugabe youth militia, Mr Tsvangirai said a free and fair poll was impossible in the current climate of violence.He called on the United Nations and the African Union to intervene to stop "genocide" in the former British colony.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.The White House said in a statement: "The government of Zimbabwe and its thugs must stop the violence now."