Zimbabwe's opposition has said it will bring the government to its knees with Kenya-style mass protests if the president, Robert Mugabe, carries through extensive plans to rig Saturday's presidential and parliamentary elections.

Mugabe has vowed to use the army to crush any demonstrations and warned Zimbabweans not to waste their votes on supporting opposition candidates whom he said he would never allow to come to power.

The 84-year-old president would struggle to extend his 28-year rule in a clean election amid widespread hunger, mass unemployment, 100,000% inflation and a currency devaluing so fast that the few people with jobs are paid in billions of Zimbabwe dollars.

Monitoring groups say the ruling Zanu-PF party has paved the way to steal the election by printing millions of extra ballots, intimidating rural voters by threatening their food supplies, permitting policemen into polling booths to "help" voters, and fixing the electoral roll.

Among those registered to vote is Desmond Lardner-Burke, who was born a century ago and, as justice minister in the white Rhodesian government, jailed Mugabe as a terrorist. Lardner-Burke died years ago in South Africa.

Ian Makoni, the election director for the opposition presidential candidate Morgan Tsvangirai, who narrowly lost the 2002 ballot amid widespread fraud, said his Movement for Democratic Change party would not repeat its mistakes of six years ago.

"The lesson from 2002 is we didn't have a plan for after the vote. Everyone stayed at home and said we will go to the courts. We have seen the lesson from Kenya. We don't want the violence that happened in Kenya. The bit I like about what happened in Kenya was they knew there would be fraud and they were ready," he said.

"We will be out on the streets celebrating when the polls close. It will be a celebration which can turn into a protest easily. Zimbabweans are angry, they are desperate, they are ready to protest. It's the tipping point we are planning for."

Mugabe told an election rally this week that a vote for the opposition would be wasted because his opponents would "never be allowed to rule this country", and he threatened to put down any Kenya-style protests.

"We have enough security forces. No nonsense, if that is what the likes of Tsvangirai are planning, they are dreaming. That will never happen here. Never ever," he said.

The most recent opinion poll gives Tsvangirai 28% of the vote, Mugabe 20% and a third candidate who broke away from Zanu-PF, Simba Makoni, 9%.

Nearly a third of those polled were undecided or declined to reveal a preference. The opposition says the bulk of them will be voting against Mugabe.

Tsvangirai said the real challenge for the opposition is to make sure its votes count. "We expect the enemies of justice to engage in every trick in the book," he said.

The opposition says one of those tricks is to have printed 9m ballots, when there are 6m names on the electoral roll – many of whom are dead, fake or improperly registered.

The law obliges the electoral commission to provide copies of the roll to the opposition in digital form. The commission handed over 80,000 printed pages scanned on to a disc, technically digital but of no use for computer programs designed to turn up multiple registrations and false identity numbers.

Nonetheless, the MDC says it has uncovered 90,000 suspicious names on the lists for 28 parliamentary constituencies, and it expects that pattern to be repeated in the other 182.

There are 25,420 registered voters in the Harare North constituency but 8,201 - nearly one third of the electorate - are listed as living in a single small area, described as Hatcliffe Housing Cooperate, with just 36 dwellings. Some houses have more than 300 people assigned to a single address.

The MDC calculates that the last presidential election was stolen through a combination of ballot stuffing and by preventing hundreds of thousands of its supporters in the cities from voting by creating huge queues with too few polling stations.

Election monitoring groups estimate that for Harare polling stations to handle every potential voter, each will have to be checked on the roll, cast four ballots and leave in half a minute.

"It will be a problem," said Makoni. "Our hope is that voters will be so angry they will just stay to make sure they vote no matter how long it takes."

Although the opposition leadership has been largely left alone since Tsvangirai was badly beaten up by Zanu-PF forces a year ago, the government has pursued election workers of the MDC and Simba Makoni by harassing and arresting them in their thousands. Some have been detained for campaigning door to door or for putting up election posters in areas already smothered with Zanu-PF propaganda.

The ruling party has also pressured hungry rural voters through village chiefs by warning them that if the count for their area favours the opposition they will lose their food supply.

Even if Zimbabweans do not bow to intimidation, there is no guarantee their votes will count. Ballots are counted in polling stations but the results announced after they are collated at a regional centre, which is where the numbers were changed in favour of Zanu-PF in the last election, according to the opposition.

Makoni said this time the MDC will use its agents to immediately announce the results from each polling station and pre-empt any alterations. It will then collate them at an election centre in Harare and issue its own count ahead of the electoral commission, which is headed by a former military office.

"In order to avoid skulduggery, our supporters will follow the documentation with the results from the polling stations and to the collation centres to protect it. It's like a river that will be swelling and then if the government tries to deny our victory it will not be able to turn it back," he said.

The opposition has in the past threatened - and failed - to mobilise millions of Zimbabweans. If anger finally overcomes fear and fatalism, protesters will face a regime that has vowed never to surrender power.

The police and army say they will crush opposition protests. The Zimbabwean justice minister, Patrick Chinamasa, said allegations of rigging were no more than an admission of defeat.

"What in fact is evident now is that Tsvangirai and his camp is now panicking. They see the crowds that our president is drawing, they see our popularity. In fact they are going to be wiped out of the political map, they are staring defeat in their face. They are now trying to find excuses to justify that defeat," he said on state television.