Mugabe hurled the insult at former finance minister Simba Makoni on Thursday in a television interview and vowed to humiliate the opposition in the March 29 general elections.

Makoni, expelled from Mugabe's ruling ZANU-PF last week, has emerged as the most serious political challenger to Mugabe in two decades, at a time when the veteran leader is struggling to convince Zimbabweans he can ease their economic hardships.

Ibbo Mandaza, a senior member of Makoni's campaign team, dismissed Mugabe's remarks as the rumblings of someone in power for far too long.

"Are you surprised by that? What we hope for is that the old man will have a nice retirement with his family because we are going to win this election," Mandaza told Reuters.

"We are not about recrimination. We are looking at the post- election period where we will give him the kind of respect and security that a founding father of this nation deserves."

Mugabe will attend an elaborate celebration of his 84th birthday and launch his election campaign in the southern border town of Beitbridge on Saturday.

The Western world sees Mugabe as a ruthless dictator, but regional African leaders look up to him as a liberation hero who still takes on the United States and former coloniser Britain.

Unlike Mugabe, Makoni has said he wants to restore ties with Western donors to rescue the economy, and analysts say he may have a much better chance at the elections than the divided main opposition Movement for Democratic Change (MDC).

Makoni, Mugabe and MDC leader Morgan Tsvangirai will stand in the March 29 presidential, parliamentary and council polls.

Mugabe has tried to deflect attention from Zimbabwe's economic meltdown by accusing the MDC of working with Western foes to oust him and destabilise the country, analysts say.

Opponents hope the world's highest inflation rate, over 100,000 percent, and shortages of basic goods will weaken Mugabe,but security crackdowns have tightened his grip on power.

In South Africa the two MDC factions repeated on Thursday that they did not expect a fair election process.

"In these circumstances we hold the firm view that the 2008 elections, which are being held under the same conditions as previous disputed elections, cannot by any stretch of the imagination yield a legitimate outcome," Tendai Biti and Welshman Ncube, senior officials from the two MDC factions, said in a statement in Johannesburg.

"We spent hours on our computers, hours researching, hours quarrelling, hours arguing ... and because we put so much into it, obviously as human beings we feel betrayed, we feel let down by the process."