Aid workers fear Zimbabwe's "desperate" situation may get even worse after President Robert Mugabe's government banned the distribution of food aid.
Some four million people - a third of the population - rely on aid after poor harvests and an economic crisis.
The UN said the move would severely undermine its aid work.
Opposition presidential candidate Morgan Tsvangirai was briefly detained by police for the second time this week and told not to continue campaigning.
The convoy of the Movement for Democratic Change (MDC) leader was stopped at a roadblock, his party said. It was then escorted to a police station in Esigodini, about 50km (30 miles) south-east of the city of Bulawayo.
"The (MDC) president has just been released but instructed to go back to Bulawayo, instead of proceeding with the campaign," said MDC spokesman Nelson Chamisa told Reuters news agency.
"The police say the instruction came from the top."
The opposition has accused the government of leading a campaign of violent intimidation ahead of the second round of presidential polls on 27 June when Mr Tsvangirai challenges Mr Mugabe.
Mr Tsvangirai was held for eight hours on Wednesday before being released, again without charges being made against him.
The MDC's secretary general, Tendai Biti, said the suspension of aid operations made the government's position clear: "It is almost as if the regime is sending out a message to the region, to the international community that it doesn't care, that it has no respect for life, it has no respect for the rule of law."
Judith Melby of Christian Aid told the BBC the ban would "make things absolutely desperate".
It could be a way to make sure there are no aid workers in rural areas to witness political violence, she said.
Allies of President Robert Mugabe say the scale of the violence has been exaggerated, while blaming much of it on the opposition.
News of the ban came as US and British diplomats were detained and reportedly threatened in northern Zimbabwe on Thursday. Officials said they had addressed an opposition rally.
UN agencies and the International Federation of the Red Cross say they are not directly affected by the ban.
But the UN distributes its food through other international agencies, so correspondents say its work is likely to grind to a halt.
It's incredibly serious - children are already suffering
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