With his rival back in the country attending the funeral of a dead activist, Zimbabwean leader Robert Mugabe fought for his political life Sunday as he kicked off his campaign for a presidential run-off.

Zimbabwe opposition leader Morgan Tsvangirai arrived home Saturday after a more than six-week absence, vowing to end the three-decade rule of post-independence ruler Mugabe in the run-off election scheduled for June 27.

In an emotional speech at the funeral of 33-year-old activist Tonderai Ndira, one of at least 40 activists the opposition say have been brutally murdered in the last two months, Tsvangirai promised not to betray his memory,

"This (murder) is clear testimony of the callousness of this regime and the extent to which they are prepared to go in order to stay in power without the support of the people," he told mourners.

"They can kill us, they can maim us, but on June 27 we are going to vote this man out. We will not betray the life of Tonderai."

Mugabe was set to deliver his first official campaign speech in Harare later Sunday in which he is expected to tear into Tsvangirai and his Movement for Democratic Change (MDC) party with his habitual fiery rhetoric.

The 84-year-old, in power since 1980, has acknowledged a "disastrous" election setback in the first round of voting that he lost to Tsvangirai. His ZANU-PF were also defeated in simultaneous parliamentary polls.

Justice Minister Patrick Chinamasa set the tone for the election campaign on Sunday when he linked the opposition to colonial-era enemies Britain and white farmers -- but he admitted that the ruling party was fighting for survival.

"We are now fighting with our backs to the wall," he told the state-owned Sunday Mail newspaper.

Despite fears of an assassination plot and the threat of treason charges, Tsvangirai returned to Zimbabwe on Saturday.

The former trade union leader defeated Mugabe in a first round of voting on March 29, but not by enough to secure an outright victory.

He had been abroad since early April lobbying regional leaders to pressure Mugabe into holding elections under the watchful eye of regional peacekeepers and election observers.

The aftermath of the disputed first-round polls, the results of which were delayed by nearly five weeks, has been marked by violence that the opposition claims is designed to rig the run-off.

Rights groups and the United Nations have said the attacks are being directed at followers of Tsvangirai's MDC movement, with pro-government militias accused of a campaign of terror in the countryside.

Activist Ndira, whose funeral was held on Sunday, was killed on May 14 after being suffocated by attackers, an opposition spokesman in Johannesburg told AFP.

Tsvangirai called Ndira, who put up posters and whipped up support ahead of opposition meetings, "a symbol of courage" and "a symbol of commitment, a commitment to the struggle to get all of us free."

On his return on Saturday, Tsvangirai made clear his position on several lingering questions about his campaign.

Firstly, he rejected the idea of a coalition government with Mugabe, which some have suggested would allow the ageing president a graceful exit and prevent further violence.

And he called for regional peacekeepers and election monitors from regional body the Southern African Development Community to be deployed by June 1.

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

Tsvangirai is threatened by a treason charge after he was accused of plotting to overthrow Mugabe with connivance from former colonial power Britain in April.

Tsvangirai, who was beaten unconscious while in police custody in March last year, has faced treason charges on two previous occasions.

He had twice announced his intention to return to Zimbabwe only to delay the move and his long absence from the country ahead of the run-off had begun to raise questions about his leadership qualities.