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.
'Desperate' situation
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.
Militias
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
The ban prevents aid workers from distributing food aid, as well as working on water, sanitation and education.
James Elder, spokesman for UN children's agency Unicef, told the BBC the ban was "incredibly serious".
At least 185,000 children around the country would not get the support they need, he added.
Some aid workers fear that pro-Mugabe militias could be the ones to implement the ban.
EU Aid Commissioner Louis Michel said he was "deeply distressed" that hundreds of thousands of people dependent on European Commission aid "now face an even more uncertain future".
Politicising food aid
The government has accused some aid groups of campaigning for the opposition and has said they should re-apply for accreditation and promise not to get involved in politics.
Christian Aid's Judith Melby spoke about the situation in Zimbabwe
Deputy Information Minister Bright Matonga told the BBC that the process of re-applying would not take long.
But Mr Elder said he feared it could take a long time - "time the children don't have".
One aid worker told the BBC's Caroline Hawley that the ban was to stop aid groups competing with the government in distributing food aid.
The authorities have been accused of only delivering food to their supporters and withholding it from areas which back the opposition.
The government has denied these accusations.
The UN's World Food Programme was planning to feed some 300,000 people this month alone. Our correspondent says many are HIV-positive and extremely vulnerable.
The opposition and donors blame Mr Mugabe's mismanagement for the country's economic crisis, widely seen as a major cause of poverty and hunger.
Just one in five adults in the country has a regular job and inflation is officially put at 165,000%.
Mr Mugabe blames the crisis on a Western conspiracy to remove him from power.
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