Mr Mugabe has been ratcheting up the rhetoric ahead of the run-off
President Robert Mugabe has vowed that the main opposition party will never lead Zimbabwe and said he was prepared to "go to war" for his country.
He is due to face Morgan Tsvangirai, leader of the Movement for Democratic Change, in a 27 June run-off poll.
Mr Tsvangirai was released after being arrested for the fifth time this week.
Meanwhile, deputy MDC leader Tendai Biti appeared in court in Harare, where a judge is to rule on the legality of his arrest on treason charges.
Mr Tsvangirai has been detained several times on the campaign trail.
On Saturday, he was stopped with 11 members of his party at a roadblock, the MDC said, and held at a police station for three hours.
The MDC issued a statement saying that it was now clearly impossible to talk about free and fair elections.
Mr Tsvangirai gained more votes than Mr Mugabe in the first round of voting in the presidential election in March, but not enough for an outright victory.
Speaking at the burial of a former independence fighter, Mr Mugabe said he would never accept the MDC taking over the government of Zimbabwe.
Robert Mugabe vows to fight anyone who 'undermines' his party
He described the opposition as "lackeys" and referred to Zimbabwe's past struggle for independence from its colonial ruler, Britain, saying the country should not be "lost" again.
"We shall never ever accept anything that smells of a delivered parcel that comes through what they call the MDC here," Mr Mugabe said at the ceremony.
"We fought for this country. Now we have it under control. After all that work, can we allow this country to be taken over by lackeys? That will never happen in our lifetime. It will never happen.
"We are prepared to fight for our country and to go to war if we lose it the same way our ancestors lost it."
Mr Mugabe stopped short of explicitly calling for war if the opposition won the run-off vote, but the remarks raised the stakes in his fight to hold on to his job, says the BBC's Peter Greste in neighbouring South Africa.
The Zimbabwean government faces growing pressure from regional leaders over the harassment of opposition leaders two weeks before the presidential run-off election.
On Friday, Botswana lodged a formal protest over the current actions of the Zimbabwean authorities.
Mr Biti was arrested on what police said were treason charges when he returned from neighbouring South Africa on Thursday. The charges carry a possible death sentence.
Tendai Biti has not yet been formally charged with treason
He has not yet been formally charged but that is expected to take place on Monday, says the BBC's Peter Greste in Johannesburg.
It took an order from the High Court for police to present Mr Biti - who had been taken to an undisclosed location - to the court and give him access to lawyers, our correspondent adds.
At the time of his arrest, national police spokesman Wayne Bvudzijena said Mr Biti would be charged with treason "for publishing a document that was explaining a transitional strategy around March 26".
He said he would also be charged for proclaiming victory in the 29 March elections before official results were published.
The US ambassador to Zimbabwe said the US was very concerned about the treason charge and did not consider it justified.
Mr Tsvangirai says more than 60 opposition supporters have been killed in political violence since the March elections and 200 more are unaccounted for.
Mr Mugabe's supporters say the scale of the violence has been exaggerated and blame the MDC for some attacks.
But human rights groups accuse the ruling party of being behind most of the violence.
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