From the Southern African Development Community (SADC), the top regional body, to former South African President Nelson Mandela, African leaders have piled increasing pressure on Mugabe to call off a presidential run-off on Friday.
Mugabe, 84, who trailed Tsvangirai for the presidency in a first round election in March, has dismissed international condemnation of violence against the opposition and has vowed to extend his 28 years in power.
Tsvangirai, who withdrew from Friday's run-off and has taken refuge in the Dutch embassy in Harare since Sunday, said in an interview with Britain's Times newspaper the time for talking to Mugabe would end if he went ahead with the election.
"Negotiations will be over if Mr Mugabe declares himself the winner and considers himself the president. How can we negotiate?" said Tsvangirai, who insists Mugabe must go so Zimbabwe can end its political turmoil and economic meltdown.
The United States said on Thursday Mugabe's government should accept the MDC's offer to talk.
"That offer obviously ought to be taken up. But it can't be taken up from a position in which the Zimbabwean authorities declare themselves the victors and then believe they can divide the spoils. That's not going to work," U.S. Secretary of State Condoleezza Rice told reporters in Japan.
If Mugabe approached him afterwards, Tsvangirai said he had this message: "I made these offers, I made these overtures, I told you I would negotiate before the elections and not after - because it's not about elections, it's about transition.
"You disregarded that, you undertook violence against my supporters, you killed and maimed, you are still killing and maiming unarmed civilians, the army is still out there.
"How can you call yourself an elected president? You are illegitimate and I will not speak to an illegitimate president."
Zimbabwe's Electoral Commission said on Wednesday that Friday's poll would go ahead.
In a separate statement, Tsvangirai denied that he had called for military intervention.
