Mr Tsvangirai had planned to address a major MDC rally in Bulawayo
Opposition leader Morgan Tsvangirai has postponed a return to Zimbabwe because of concerns about a plot to kill him.
"We have received information from a credible source concerning a planned assassination attempt," his spokesman George Sibotshiwe said.
Mr Tsvangirai was set to return to campaign for a run-off presidential election against Robert Mugabe.
On Friday, the US ambassador warned post-election violence had made a fair second round run-off vote impossible.
James McGee told the BBC he had evidence that the police and military had been involved in "pure unadulterated violence designed to intimidate people from voting" in the election, which the electoral commission has set for 27 June.
Opposition and human rights groups have said hundreds of opposition supporters have been beaten up and at least 30 killed since the first round on 29 March.
According to official results, Mr Tsvangirai won the presidential poll, but not by enough to avoid a run-off with President Robert Mugabe. He has insisted he did pass the 50% threshold and so should have been declared the outright winner.
After spending more than a month outside Zimbabwe since then trying to drum up international support, Mr Tsvangirai had been planning to return to Harare on Saturday and resume his campaign to oust Mr Mugabe.
We can't say why he will not be coming today, except to say it's due to circumstances beyond our control
Nelson Chamisa MDC spokesman
He had been due to speak to newly-elected members of parliament from his party, who will form a majority for the first time since independence.
The MDC leader had also planned to address a major rally in Zimbabwe's second city, Bulawayo, on Sunday.
The rally is still scheduled to go ahead, but Mr Tsvangirai's spokesman said the politician's return had been postponed indefinitely.
Last year, Mr Tsvangirai was treated in hospital after being assaulted by police.
The BBC's Caroline Hawley in Johannesburg says that despite a campaign of state-sponsored violence and intimidation against MDC supporters, Mr Tsvangirai has no choice but to contest the run-off or allow Mr Mugabe to win by default.
Ambassador McGee warned that such "politically-inspired" violence cast doubt on whether a free and fair election could take place.
"Too many people have been killed, too many people have been maimed, too many people have been dislocated from their homes," Mr McGee told the BBC.
He said the attacks involved "mainly beatings to the back and buttocks, we've seen quite a few broken limbs, we've seen cuts to the head".
Mr McGee said he had met victims on a trip with British, Japanese, EU, Dutch and Tanzanian diplomats, during which he said they were harassed by police.
Along with so-called war veterans, he said they had evidence "police and military are involved in these attacks".
It was "pure unadulterated violence designed to intimidate people from voting in the next election", he said.
Deputy Information Minister, Bright Matonga, insisted that the Zimbabwean government did not support any violence, whether by MDC or Mr Mugabe's Zanu-PF.
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