Zimbabwe's main opposition leader has said there has been an "upsurge" of support calling for change in Saturday's elections.
Opposition rallies have been allowed in rural areas
But Morgan Tsvangirai said he was wary of "manipulation", saying winning the vote did not mean winning power.
Meanwhile, cabinet minister Paul Mangwana has told the BBC that Mr Mugabe would accept defeat, although he said it was "highly unlikely".
"He is a democrat," Mr Mangwana said, pointing to a 2000 referendum defeat.
Earlier, a former Mugabe loyalist spoke openly about the possibility that the president might be defeated in poll.
Former Education Minister Fay Chung told the BBC that the two main rivals could win if voters were "courageous enough to come out in large numbers".
The independent presidential candidate, former Finance Minister Simba Makoni, meanwhile complained he had been unable to place adverts in the state media.
In an interview with the BBC, Mr Tsvangirai said he was impressed with the level of support he had received, particularly in rural areas, while campaigning for this weekend's local, Senate, House of Assembly and presidential elections.
"I have no doubt that we have overwhelming support. In fact, the Zanu-PF may actually find it difficult to suppress... the upsurge on the ground."
Nevertheless, the leader of the Movement for Democratic Change (MDC) said he remained pessimistic about translating popular support into power.
"This is the crisis we faced in 2002 and 2005," he said. "Although we had the support of the people, we didn't manage to win the election."
"This manipulation undermines the will of the people. This time around, we hope that we are able to mitigate against some of the possible manipulation."
Meanwhile, Mr Tsvangirai's pilot has been charged with fraud, for checking into a hotel room booked in someone else's name, the MDC says.
MDC treasurer Roy Bennett says this has hampered Mr Tsvangirai's ability to address rallies in rural areas.
A majority of Zimbabwe's voters live in rural areas, where Mr Mugabe has traditionally enjoyed his strongest support.
The US and the EU have accused Mr Mugabe of rigging previous elections - charges he has denied. Western monitors have been barred from this election.
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Mr Tsvangirai said that if he was elected president, the people of Zimbabwe would be given the chance to rewrite their constitution.
He also called for new policies to address Zimbabwe's economic crisis, which has seen the country's annual inflation rate hit 100,000% - the world's highest.
Earlier, a former government minister told the BBC that Mr Mugabe might not be able to prevent one of the two main opposition candidates from winning the presidential election.
"I think the issue is whether the electorate is going to be brave or courageous enough to come out in large numbers, because I think the rigging of the election has been possible when there were small numbers dividing votes," Ms Chung said.
"But if you have millions coming out to vote, it will be very difficult to rig. If the polling agents and the observers are very watchful, it will become more difficult."
Ms Chung, a Senatorial candidate now allied to Simba Makoni, conceded that "there are a lot of 'ifs'", but insisted that Zimbabweans were desperate for change after 28 years of Mr Mugabe.
"I hear people saying... 'We are being abused. If we keep on electing the same government, we will continue to be abused'," she added. "So the question is: will they vote for the MDC or Simba's movement?"
Ms Chung said Mr Makoni's decision last month to stand against the president because of what he described as a "failure of national leadership" had begun a "period of change" in Zimbabwe.
"The steps taken by Simba Makoni have broken a Gordian Knot in which we were so tightly strung that we did not know how to get out of it," she told Radio 4's World Tonight programme.
"I think that whether he wins or not - I think he will win - he has changed the political geography of the country."
Ms Chung also acknowledged there was a potential for violence similar to that witnessed after the Kenyan presidential election last year, regardless of the result.