Tsvangirai told a news conference in Harare that Zimbabwe was being run by a military junta and 66 supporters of his Movement for Democratic Change (MDC) had been killed since disputed March elections.
But he rejected calls for the June 27 run-off against President Robert Mugabe to be cancelled because of the violence.
"Following the announcement of the date for the run-off, no-one can change that due process unless Robert Mugabe concedes defeat, or collapses. It therefore means that a government of national unity negotiated before the runoff does not arise," Tsvangirai said.
Ruling ZANU-PF defector and former finance minister Simba Makoni said earlier the poll must be called off because a free and fair vote was impossible.
Makoni's statement came after a similar call by U.S.-based Human Rights Watch, which said brutal intimidation and murder by supporters of President Robert Mugabe made normal campaigning impossible.
"This country is effectively being run by a military junta, 66 people have been killed and 200 unaccounted for," Tsvangirai said.
An EU-U.S. summit in Slovenia on Tuesday called on the Zimbabwe government "immediately to cease the state-sponsored violence and intimidation against its people."
It urged U.N. Secretary-General Ban Ki-moon to send monitors to Zimbabwe to deter further violence.
Makoni, who challenged Mugabe in disputed March 29 elections, told reporters in Johannesburg that Tsvangirai must negotiate a five-year transitional government.
