"We are not in the habit of cheating. We don't rig elections.''
"I cannot sleep with my conscience if I have rigged," Mugabe said as he cast his vote in the capital Harare.
Mugabe is facing his toughest test as president since coming to power in 1980, as the country struggles under an economic crisis.
Zimbabwe has the highest rate of inflation in the world - over 100,000 per cent - and 80 per cent of the workforce is unemployed.
Police said that the country was mostly calm during voting.
However, the house of a Zanu-PF parliamentary candidate in Bulawayo was bombed early on Saturday, in the first violent incident during the election.
"Early this morning a house in Emakhandeni ... constituency was bombed. No arrests have been made as yet and no one was injured," Wayne Bvudzijena, chief police spokesman, said.
Haru Mutasa, Al Jazeera's correspondent in Bulawayo, said: "The lady in question says she doesn't know who is responsible. People don't want to speculate right now.
"The people I spoke to in the voting line are telling me that they are going to vote in protest - they don't actually think their vote is going to matter."
Mutasa said some opposition supporters are also concerned that the potential voter base against Mugabe is being split between Tsvangirai and Makoni, to the benefit of Mugabe and Zanu-PF.
Supa Mandiwanzira, Al Jazeera's correspondent in Harare, said: "The situation is very different to 2000 and 2002 where in [the] opening of the polls we had queues a kilometre long."
He said there was no indication of a heavy police presence but officers were watching polling stations "making sure people are orderly and there is no chaos".
According to Zimbabwe's electoral commission, 5.9 million people were eligible to vote.
But the Movement for Democratic Change (MDC) opposition party revealed documents leaked from the state security printer showing nine million ballot papers ordered by the commission.
Tsvangirai, leader of the MDC, alleged that the surplus ballot papers would be used to rig the vote.
Tsvangirai said he expected to win despite widespread vote-rigging.
"Victory is assured in spite of the regime's attempt to subvert the will of the people," Tsvangirai said as he cast his vote in Harare on Saturday.
"Even if the MDC wins, the election can't be said to be free and fair," he said.
Zimbabwean security chiefs said on Friday that they were ready to confront any violence that may follow the elections.
Augustine Chihuri, police commissioner, said anyone who harboured "evil" intentions would face the full force of the law.
"Those who have been breathing fire about the Kenyan-style violence should be warned that violence is a poor substitute for intelligence", he said.
Mugabe himself warned his opponents to not even "dare" think about resorting to violence in the event of his victory.
|Mugabe, who is seeking a sixth term in office, |
casted his vote in Harare [AFP]
Tsvangirai told Al Jazeera that there should be no violence on behalf of his party but could not give any assurances as to how the people of Zimbabwe would react.In their first joint statement, Tsvangirai, Makoni and Arthur Mutambara, leader of a breakaway MDC faction, said on Thursday that an independent scrutiny of voters' lists showed severe discrepancies.
Makoni, who is standing as an independent candidate, spoke for all three groups opposing Mugabe.
Makoni has expressed his own doubts about the fairness of the poll.
However, on Saturday he told reporters that his chances of success are "very good" and that his final tally should be "more than" his initial prediction of 72 per cent.
Tsvangirai has urged voters to remain at polling stations after casting their ballots, in order to protect their votes.
He also called on public servants not to defraud voters.
"Mugabe cannot rig elections by himself. If someone tells you to falsify the results of the elections, ignore the instructions ... because it is unlawful," he said.
The opposition said the voter registration list shows that between December 2007 and February 2008 the number of new voters increased by up to 11 per cent in sparsely populated rural areas, where the ruling Zanu-PF holds most power.
This compares with a 2 per cent increase in urban opposition strongholds, the oppostion says.
"This is a five-fold difference which is not supported by our urban and rural demographic profile. We don't understand the discrepancy," Makoni said.
State media predicted a day before the poll that Mugabe would win an outright majority in the first round of voting, thus obviating the need for a run-off within three weeks.