Zimbabwe's rival opposition camps would form a united front against President Robert Mugabe if he is forced into a run-off by Saturday's election, a top official with one of the campaigns said on Wednesday.

Mugabe faces an unprecedented challenge in the ballot from Simba Makoni, a former ruling party ally, and old rival Morgan Tsvangirai. Both accuse Mugabe of wrecking the once prosperous African country but have so far dismissed talk of a coalition.

Makoni's national campaign coordinator said they would join forces, however, if Mugabe fails to win the outright majority he needs to avoid a second round.

"It's an automatic," Nkosana Moyo told reporters in Johannesburg. "Zimbabweans would like to see an end to Mugabe's mismanagement, so any configuration that leads to a run-off will see Mugabe on one side and everybody else on the other."

Moyo said there were understandings with Tsvangirai's main faction of the Movement for Democratic Change (MDC) on forming a national unity government if Mugabe lost. There was no immediate comment from the MDC.

Makoni, expelled from Mugabe's ZANU-PF party last month, and Tsvangirai, have promised to tackle the crisis marked by chronic food and fuel shortages, a virtually worthless currency and inflation of more than 100,000 percent.

But Mugabe, who blames economic woes on sabotage by his Western foes, says he and his ZANU-PF are braced for victory.

Mugabe has boasted during the campaign that the opposition MDC will never be in power as long as he is alive and told Al Jazeera on Tuesday that he was "overconfident" of winning the election.

Opponents, who accuse Mugabe of rigging past elections, say such comments reinforce their fears that the vote will not be fair. Military chiefs have said they would never accept a Mugabe defeat.

Mugabe and his officials have been slapped with sanctions by Britain, the United States and other Western countries for cracking down on opponents and alleged human rights abuses.

The opposition campaigns have already raised what they see as election irregularities, pointing to reports that millions of excess ballots have been printed as well as plans to have police assist voters in polling booths.

Moyo said the lack of media coverage of the opposition campaigns, a flurry of late voter registrations and the failure of election authorities to properly educate Zimbabweans were further concerns that could mar the polls.

The presidential election is being held alongside parliamentary and municipal polls.