'Plot' delays Tsvangirai's return to Zimbabwe
"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.
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.
A controversial shipment of arms from China and destined for Zimbabwe has arrived in Harare, the Weekender newspaper reported on Saturday -- apparently thanks to assistance by the South African government.
The report said the Zimbabwean government had confirmed that three million rounds of assault rifle ammunition, 3 000 mortar rounds and 1 500 rocket-propelled grenades -- ordered from the Chinese government -- had arrived in Harare.
The South African government has denied media reports that it assisted in the delivery of the arms by fuelling the Chinese vessel, the An Yue Jiang, that was transporting the arsenal.
There are fears that President Robert Mugabe is planning to use force to storm back to power in Zimbabwe's presidential run-off election to be held on June 27. He has deployed the army, police and intelligence units across Zimbabwe to campaign for him through intimidation and coercive tactics, the report said.