The president drew fresh condemnation after issuing his latest warning to white farmers to surrender their properties - a process he insisted will have to be completed by the end of this month.
The white farmers group Justice for Agriculture (Jag) said that confiscating the land in this way could result in a "humanitarian disaster".
Their comments were endorsed by Washington, where the State Department condemned Mr Mugabe's latest threats.
"At a time when six million Zimbabweans are without food supplies, that the government attempts to evict commercial farmers and thousands of farm workers is extremely reckless - and in fact reprehensible," said spokesman Philip Reeker.
The Zimbabwean Government says the evictions are necessary to redress the imbalances which occurred under British colonial rule, when 70% of the country's best farmland became concentrated in white hands.
'Violence and hatred'
Mr Mugabe's speech was to mark Heroes' Day - which celebrates victory over the old white regime.
Zimbabwe's land reform
Opposition leader Morgan Tsvangirai said the president had turned the occasion into an "indecent partisan junket to spread a message of violence and hatred".
"Zimbabwe is currently suffering under the effects of Mugabe's dictatorship with millions facing death from disease, starvation and state-sponsored violence," he said.
"Yet Mugabe's message to the nation was a promissory note for more misery and death."
Aid agencies have already predicted that up to 13 million people in six southern African countries, about half of them in Zimbabwe, face starvation by February as a result of drought and political mismanagement.
The seizure of white farms in Zimbabwe is widely believed to have exacerbated the country's food crisis, causing agricultural upheaval at an already difficult time.
Mr Mugabe also stands accused by aid agencies of diverting food aid away from opposition dominated areas into the mouths of his own followers - allegations he fiercely denies.
Nearly 3,000 farmers have been told to leave their properties, but so far only about 500 have left despite threats of fines or jail for ignoring the eviction order.
Mr Mugabe said the August deadline would allow new owners of the land enough time to prepare and plant for the new crop season in October.
Jenni Williams of Jag told the BBC that Zimbabwe's white farmers accepted that land must be redistributed, but not at the expense of production.
The government had offered nothing that would help avert the threatened famine, she said.
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