Fears of an assassination plot prevented Zimbabwe's opposition leader Morgan Tsvangirai from taking part in the start of his election campaign on Sunday ahead of a presidential run-off next month.

As his party prepared the first rally of his latest push for the presidency, which he had been scheduled to address, Tsvangirai was set to stay in South Africa because of fears for his life should he return, his party said.

The former trade union leader, who will face veteran President Robert Mugabe in a June 27 second-round poll, has been out of the country for more than a month after winning a first-round of elections at the end of March.

After several delays, he had been finally expected back on Saturday but pulled out at the last minute after his party said they had discovered via a credible source that an assassination plot had been mounted against him.

"There are no plans for him to leave today," said a party spokesman on Sunday from Johannesburg, where Tsvangirai remains after arriving back on Saturday from a trip to Northern Ireland.

His party was to hold its first rally ahead of the run-off election in the southern Zimbabwean city of Bulawayo.

Asked when Tsvangirai might return, the spokesman said: "It is still about assessing the options and making sure that things on the ground are ready for his return from a security perspective."

Violence has rocked Zimbabwe since a first round of elections in March in which Tsvangirai defeated veteran President Robert Mugabe, but not by enough to secure an outright victory.

Pro-government militias have since been accused of harassing and killing opposition supporters.

As well as the alleged assassination plot, Tsvangirai faces other dangers in his homeland, namely the threat of a treason charge after being accused of plotting with erstwhile colonial power Britain to overthrow the government.

Mugabe, Africa's oldest leader, acknowledged on Friday that his loss in the first round of voting in March had been "disastrous" but he began campaigning on Saturday for his re-election with advertisements in state media.

The 84-year-old, who has ruled the former British colony since independence in 1980, lost the first round by 43.2 percent to 47.9 percent against Tsvangirai and now is fighting for his survival after nearly three decades in power.

The election process has been marred by delays, violence and allegations of electoral fraud. Economic woes deepen by the day, with official inflation at 165,000 percent and unemployment of 80 percent.