Spokesman George Sibotshiwe told The Associated Press that "we have received information from a credible source concerning a planned assassination attempt."

He said there was no immediate clue about who was behind the alleged plot to kill the Movement for Democratic Change leader.

Tsvangirai, who had been out of the country for the past few weeks, had planned to take part in a rally Sunday ahead of the June 27 presidential runoff. He won the first round of voting against President Robert Mugabe but fell short of an outright majority.

Mugabe has ruled since independence from Britain in 1980 and once was hailed for promoting racial reconciliation and bringing education and health care to the black majority. But in recent years he has been accused of becoming authoritarian and pursuing policies that wrecked a farm-based economy that had been the thriving breadbasket of southern Africa.

The opposition and most observers say that the mounting violence and intimidation, mainly targeting opposition supporters, make it virtually impossible to hold a credible runoff election.

Sibotshiwe said party officials would consult their own security experts and regional leaders for advice. He said Tsvangirai would return "at the earliest opportunity."