Zimbabwean leader Robert Mugabe called for unity in his faltering party on Sunday ahead of run-off elections next month in a fiery speech in which he also threatened to expel the US ambassador.

Speaking to a crowd of 2000 at his party's headquarters, he launched his election campaign with a series of attacks on newly returned opposition leader Morgan Tsvangirai whom he likened to a frog and described as a coward.

Now fighting for his political life after losing a first round of voting in March, he lambasted foreign "imperialists" who wanted to influence Zimbabwe and took aim at US Ambassador James McGee whom he accuses of interferring.

"Tall as he is, if he continues doing that (meddling in our affairs), I will kick him out of the country," the 84-year-old leader said as he stood in front of giant banner showing him with a raised fist.

The independence war hero, in power since 1980, has acknowledged his loss a in a first-round poll in March was "disastrous" and his justice minister said on Sunday the party was "fighting with our backs to the wall."

'A formidable fight'

"Disunity, that's what is killing us," Mugabe said, as he called for a new push to win the second round run-off scheduled for 27 June.

"We should gear ourselves for a formidable fight and that means a fight that is waged through our support."

He faced a challenge from ex-finance minister and former party loyalist Simba Makoni in the first round who polled about eight percent.

Zimbabwe opposition leader Morgan Tsvangirai arrived home on Saturday after a more than six-week absence during which he lobbied regional leaders to put pressure on Mugabe.

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."

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

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.

Ambassador angers the regime

US Ambassador McGee angered the regime on 13 May when he organised a tour of hospitals with other Western diplomats to see victims of political violence.

"I think it is absolutely urgent that the entire world knows what's happening in Zimbabwe," James McGee told an AFP correspondent afterwards.

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 (Sadc) to be deployed by 1 June.

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 citing fears of an army assasination plot.

Sapa