Zimbabwe's main opposition party has accused the government of printing millions of surplus ballot papers for the presidential and legislative polls.
The opposition say Mr Mugabe is driving the economy into the ground
The Movement for Democratic Change (MDC) says leaked documents show nine million papers have been ordered for the country's 5.9 million voters.
But, the head of the electoral commission rejected suggestions that the extra papers might be misused.
Judge George Chiwese told the BBC that the vote would be free and fair.
President Robert Mugabe has accused the MDC of treasonous links with Britain.
OPPOSITION POLL CONCERNS
Surplus ballot papers printed
Presidential votes counted centrally
Tens of thousands of "ghost voters"
Police allowed inside polling stations
More polling stations in rural areas
State media bias
Food aid only given to Zanu-PF supporters
Chiefs used to campaign for Zanu-PF
Speaking ahead of the 29 March election, he said the opposition would never take power as long as he was alive.
The MDC's secretary general Tendai Biti said the claims of excess ballot papers were based on leaked documents from the government's security printers.
The MDC also says 600,000 postal ballots have been ordered for a few thousand police, soldiers and civil servants.
While ballots are reported to have been ordered for police and military personnel and civil servants living away from home, about four million Zimbabweans living abroad are not permitted to vote by post.
Mr Biti said 84-year-old Mr Mugabe, who is seeking a sixth term in office, faced a no-win situation.
Bigger crowds are being seen at opposition rallies
"The credibility gap will be so huge," he said. "If he steals the election he will get a temporary reprieve but that will guarantee him a dishonourable if not bloody exit."
He added that the president would probably be forced out of office in the weeks following the elections by the deepening economic crisis and the shortage of basic public services.
Mr Mugabe has been in power since independence from Britain in 1980.
He is being challenged in presidential polls by former finance minister Simba Makoni, who is running as an independent, and the MDC's Morgan Tsvangirai.
Analysts say the election poses the biggest threat to his rule since he took office. If Mr Mugabe fails to secure 50% of the vote there will be a second round.
Mr Tsvangirai told an estimated 30,000 in Harare on Sunday that he expected Mr Mugabe to "engage in every trick in the book" to win the polls.
HAVE YOUR SAY
It's a country without hope with a dictator who will stop at nothing to remain in power.
Barry Verona, Ex pat - now Canada
One music group sang to applause: "Saddam has gone, Bob is next."
Mr Mugabe told some 10,000 supporters in Harare on Saturday he remained confident of victory.
"Tsvangirai will never, never rule this country," Mugabe said.
"Those who want to vote for him can do so but those votes will be wasted votes."