Police have told the MDC they cannot guarantee its leaders' safety at rallies
Zimbabwe's authorities have stopped opposition presidential candidate Morgan Tsvangirai from campaigning for the 27 June election.
The order banning "several future rallies" came after police briefly detained Mr Tsvangirai ahead of a rally in the second-largest city of Bulawayo.
The length or extent of the ban, which cites security fears, is not yet clear.
It comes soon after the government banned food aid distribution, saying agencies were helping the opposition.
Relief organisations reject the charges, warning that Zimbabwe's "desperate" situation could get even worse. They had been hoping to feed around 600,000 people this month, as the country has just had its harvest.
But when that food runs out early next year, they say between two and four million people - up to a third of the population - will need food aid.
The restrictions on aid agencies - making the government the sole provider of food aid - have drawn widespread condemnation.
The US ambassador to Zimbabwe said Mr Mugabe's government was using food as a weapon to get votes, distributing food only to its own supporters.
He said the government was also confiscating the identification cards of opposition supporters which means they cannot get government food aid and will not be able to vote in the presidential run-off.
"We are dealing with a desperate regime here which will do anything to stay in power," said Ambassador James McGee.
White House spokeswoman Dana Perino said the ambassador was considering what further action to take to put pressure on the Mugabe government.
The UN High Commissioner for Human Rights, Louise Arbour, said the decision to prevent the agencies carrying out their work was "a true perversion of democracy".
UN humanitarian chief John Holmes said it was "deplorable" and urged the government to "reconsider and rescind the decision as soon as possible".
Zimbabwe's National Organisation of Non-Governmental Organisations (Nango) said the ban on field operations was illegal and would have an "immediate, critical and negative impact especially on children, people living with HIV/Aids, the elderly, pregnant mothers and the disabled".
Mr Tsvangirai's Movement for Democratic Change (MDC) says gatherings planned for Harare's high density townships of Glen Norah, Kambuzuma and Mufakose and the city of Chitungwiza were also banned by police.
The party says the ban is "rank madness" as the meetings are its only way to communicate with supporters because it is denied access to public media.
In a statement, the party quoted a letter from the police saying that "because the MDC had complained that its leaders were targets for assassination the authorities could not guarantee their safety and were therefore banning several future public rallies".
The government has previously dismissed MDC concerns of a possible assassination threat as fantasy.
The MDC accuses President Mugabe's supporters of leading a campaign of intimidation which has forced thousands from their homes and left at least 65 dead.
Mr Tsvangirai was detained by police for several hours on Friday - the second such incident in three days. On Wednesday, he was stopped and held for eight hours before being released without charge.
Zimbabwe's information minister Sikhanyiso Ndlovu declined to comment.
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