Zimbabwe opposition leader Morgan Tsvangirai was detained by police Wednesday, his spokesman said.
Tsvangirai and about 14 party officials were being held at a police station in Lupane, north of Bulawayo, Zimbabwe's second-largest city, said George Sibotshiwe, Tsvangirai's spokesman.
Tsvangirai was campaigning in the area when his convoy was stopped at a roadblock, Sibotshiwe said.
No charges have been filed, Sibotshiwe said.
Despite threats on his life, Tsvangirai returned to Zimbabwe 12 days ago to face President Robert Mugabe in a presidential runoff on June 27.
No comment from police was immediately available.
Nelson Chamisa, a spokesman for the opposition Movement for Democratic Change, said Tsvangirai had planned on spending the week campaigning in Western Matabeleland.
At least two of Tsvangirai's rallies since his return May 24 have been banned on security grounds by police, who said they could not guarantee his safety.
Rights groups have criticized the violence and intimidation in the run-up to the vote.
At least 58 opposition supporters have been killed. Thousands have been driven from their homes, especially in rural areas, where Mugabe lost much support in the first round of voting.
Meanwhile, Human Rights Watch said Wednesday that the Zimbabwe government is imposing control over food aid to intimidate voters before the presidential runoff.
The New York-based organization called for the government to lift restrictions on international aid groups operating in Zimbabwe.
The aid organization CARE International has been ordered to halt operations pending an investigation of allegations it was campaigning for the opposition. CARE denies that was the case.
Other aid groups have also been told to curb activities in Zimbabwe and there is concern the decision will hinder the delivery of food to millions of people.
"The decision to let people go hungry is yet another attempt to use food as a political tool to intimidate voters ahead of an election," said Tiseke Kasambala, Zimbabwe researcher at Human Rights Watch. "President Mugabe's government has a long history of using food to control the election outcome."
CARE International provides aid to about 500,000 Zimbabweans and had planned to resume food distribution this month to about 1 million people.
Critics blame Mugabe's land reform policies for the country's economic collapse, which has left Zimbabwe struggling to feed its people.
Mugabe has accused the West of using non-governmental organizations to channel funds to the opposition.
Kasambala called on the government to let aid agencies carry on with their work.
"It's imperative that the government ensure that all those in need receive food irrespective of political affiliation," she said.
Human Rights Watch said a number of aid agencies working in Binga, Chipinge, Chimanimani, and Nyanga districts have also been forced to halt operations, "indicating that the government is imposing its control over food aid countrywide."
The UN High Commissioner for Human Rights, Louise Arbour, said in Rome Wednesday that she was "deeply concerned" by the reports.
"If true, this would be an unconscionable act," she said. "To deprive people of food because of an election would be an extraordinary perversion of democracy, and a serious breach of international human rights law."
Save the Children has had operations shut down in Binga but was continuing other work, an official for the organization said on condition of anonymity because the matter was sensitive.
The official said aid groups have been told by district administrators in various areas to stop operations but that "it was not all NGOs, not all areas and not all activities."
Caroline Hooper-Box, a spokeswoman for Oxfam, said food shortages of 1.4 million tons are predicted, which will put many Zimbabweans in dire need.
The White House said Wednesday that the Bush administration is "saddened and troubled" by Zimbabwe President Robert Mugabe regime's decision to suspend apolitical non-governmental operations in Zimbabwe, including those by CARE.
"The government's actions mean that 110,000 Zimbabweans dependent on CARE's assistance will go hungry this month," White House press secretary Dana Perino said.
She said it further displays the regime's "callous indifference" to the Zimbabwean people's plight and pleas for change.
"Instead of delivering empty rhetoric in Rome, where Mugabe went to participate in a world food conference, we urge the Mugabe regime to take real action, including the reversal of this ill-advised decision, to prevent government-induced starvation in Zimbabwe," Perino said.