Mugabe’s government has banned at least one international aid group from operating in the country for allegedly campaigning for the opposition.
All operations of Care International are now suspended, pending an inquiry into the claims.
Care denies that it “has encouraged or tolerated any political activity”.
The ban comes as Zimbabwe prepares for a run-off election between President Robert Mugabe and opposition leader Morgan Tsvangirai on 27 June.
Meanwhile, senior opposition figure Arthur Mutambara has been freed on bail, after being arrested on Sunday over a newspaper article.
After being released, he accused the government of a crackdown on opposition supporters ahead of the run-off – similar claims to those he made in his article.
“What has happened is nothing compared to what the people of Zimbabwe are experiencing. Mugabe is violating the human rights of our people,” Mr Mutambara said.
Mr Mugabe is currently attending the UN’s food summit in Rome, where the UK and Australia have called his presence “obscene”.
They blame him for destroying Zimbabwe’s once thriving economy.
Mr Mugabe in turns says a Western plot and poor rains are behind the hunger in his country.
Social Welfare Minister Nicholas Goche has been quoted as saying that other aid agencies could also have their activities halted.
Mr Mugabe is attending a UN summit on easing global food shortages
“Several other non-governmental organizations involved in food distribution in Manicaland province will also be asked to cease operations while we investigate them,” he told the ZimOnline website.
“There is a crucial run-off coming and our information indicates that NGOs are involved in plans to undermine our candidate.”
An official from another aid agency told the BBC News website that his group had been ordered to cease work in parts of Manicaland.
He said the agency had scaled back its operations during the elections after similar comments from ministers in the past.
Care says it has strict rules against getting involved in politics.
“Care has requested but has not yet received any details of the allegations, including names, dates and locations,” the agency said.
Care says some 500,000 Zimbabweans were affected by the suspension. Some four million Zimbabweans – a third of the population – are believed to need food aid.
“Care has pledged to co-operate with the [Zimbabwean] government in resolving the situation, so the humanitarian operations may resume as soon as possible.”
Kenneth Walker, Care’s Africa communications manager, told the BBC News website that the group’s operations were suspended last Friday.
Mr Walker said Care had been accused of being involved in the distribution of brochures on behalf of Mr Tsvangirai’s Movement for Democratic Change (MDC) and threatening to withhold food from those not supporting the opposition.
Care is involved in a number of projects in Zimbabwe, including distributing food aid, water and sanitation, micro credits and support for orphans.
The MDC has frequently accused the government of denying food aid to opposition supporters – accusations denied by the authorities.