Security fears delayed the long-awaited homecoming of Zimbabwe's opposition leader on Saturday ahead of an election showdown with veteran President Robert Mugabe scheduled for June 27.

After more than a month out of the country due to fears for his safety, Tsvangirai had signalled his return for Saturday, but security concerns again held him up.

"We are still trying to work on it," Tsvangirai's spokesman George Sibotshiwe told AFP from Johannesburg, saying the leader would not arrive at his scheduled time of 1:00 pm (1100 GMT) but his return was still possible.

A spokesman in Harare, however, ruled this out for the former trade union leader, who faces the threat of a treason charge after being accused of plotting with former colonial power Britain to bring about regime change.

Tsvangirai beat Mugabe in a first round of polling in March -- but not by enough to secure outright victory -- and Mugabe acknowledged on Friday that he had suffered an electoral disaster.

Critics of Tsvangirai say he has lost momentum since his victory in the March ballot and his adversary, Africa's oldest leader, began his campaigning on Saturday for the second-round run-off scheduled for June.

"I thank you for voting in peace. Vote for R.G. Mugabe," said an advertisement in The Herald newspaper. In a small box was his campaign theme: "100-percent total empowerment, independence."

The 84-year-old Mugabe, 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 is now fighting for his survival.

A police spokesman, Oliver Mandipaka, said the fears raised by the opposition about Tsvangirai's safety were "calculated to raise unnecessary alarm."

"Everyone in Zimbabwe is free to move anywhere they like," he told AFP.

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

Tsvangirai, who had said last weekend he would return within a couple of days, has made a series of demands to ensure a free and fair run-off election, including the presence of foreign peacekeepers and election monitors.

Foreign Affairs Minister Simbarashe Mumbengegwi told The Herald on Saturday that "there would be no further invitations" for election monitors despite pressure from Western countries.

No Western monitors were allowed to oversee the first ballot and teams from the Southern African Development Community (SADC) and the African Union (AU) were widely criticised for giving it a largely clean bill of health.

On Friday, an independent home-grown network of monitors that observed the first round said dozens of its activists had since been assaulted by suspected ZANU-PF supporters since the March 29 election.

Speaking Friday, Tsvangirai had promised to return to Zimbabwe to stand in solidarity with his supporters who, according to a raft of reports, have faced intimidation and violence from pro-government militias since the first poll.

"Mugabe lost that first round, 57 percent of the people who cast their vote did not vote for him," he said from Belfast, Northern Ireland.

"I am so confident that in spite of the violence, come the second round they will reconfirm that rejection."

Despite numerous reports from human rights and civil society groups in Zimbabwe stating the contrary, Mugabe has accused the opposition Movement for Democratic Change (MDC) of fomenting the post-election violence.

"The MDC and its supporters are playing a very dangerous game," he told leaders of his ZANU-PF party on Friday.

Zimbabwean doctors, unions and teachers have reported a campaign of terror conducted by pro-government militias in rural areas against supporters and activists of the MDC since the March elections.

The MDC says at least 32 of its supporters have been killed in the unrest.

These reports have been bolstered by the United Nations, whose representative to Zimbabwe said the majority of violence had been directed at MDC supporters, and rights group Amnesty International.