Zimbabwean security chiefs said on Friday that they are ready to confront any violence that may surround 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 and that it is a monster that can devour its creator, as it is blind and not selective in nature," he said.
Mugabe himself warned his opponents to not even "dare" think about resorting to violence in the event of his victory.
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.
|Mugabe is running for a sixth term as |
president of Zimbabwe [AFP]
Makoni, who is standing as an independent candidate spoke for all three groups opposing Mugabe.
"There is a very well thought out and sophisticated plan to steal the election from us," he said.
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 party 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 on Friday that Mugabe would win an outright majority in the first round of voting, thus negating the need for a run-off within three weeks.