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."
