Zimbabwe's opposition Movement for Democratic Change said its leader Morgan Tsvangirai was detained at a police roadblock on Wednesday in an attempt to derail his presidential election campaign.

"They are saying there's a commanding officer whom we should wait for. They are not saying why they are holding us up. It's not an arrest but illegal detention," said Tsvangirai's spokesman, George Sibotshiwe.

"It appears they want to disrupt our campaign programme."

The police were not available for comment.

Sibotshiwe said police stopped Tsvangirai's convoy and searched vehicles and passengers' documents.

MDC spokesman Nelson Chamisa said Tsvangirai was later taken to a police station in Lupane, southwest of Harare, but had not yet been charged.

Tsvangirai, who has been arrested several times in the past, defeated President Robert Mugabe in a March 29 presidential election but failed to win the absolute majority needed to avoid a second ballot. The run-off is scheduled for June 27.

Mugabe's vow never to allow the MDC to take power has stoked opposition fears that the ruling ZANU-PF will use intimidation and vote-rigging to extend the veteran ruler's 28-year rule.

The MDC says 50 people have been killed by Mugabe's supporters since the election. On Wednesday it said soldiers and ZANU-PF activists had beaten and threatened to shoot Zimbabweans who wanted to meet and support Tsvangirai.

"Mugabe is determined to turn the whole country into a war zone in order to subvert the will of the people and steal the June 27th election by any means possible," Tsvangirai said while campaigning in Bulawayo.

Mugabe and his ZANU-PF say the opposition is responsible for violence.

FOOD AS A WEAPON

Mugabe's government confirmed on Wednesday that it had suspended the operations of aid agency CARE International and other humanitarian groups, charging that they had become involved in politics and were backing Tsvangirai's campaign.

CARE International has denied the charge.

U.S.-based group Human Rights Watch accused Zimbabwe of using food as a political weapon and urged it to reverse the suspensions.

The United States said it was saddened by the decision, saying it would mean that more than 100,000 Zimbabweans would go hungry this month.

"This further displays the regime's callous indifference to the Zimbabwean people's desperate plight and pleas for change," White House spokeswoman Dana Perino told reporters in Washington.

Zimbabwe's once prosperous agricultural sector has collapsed since 2000, when Mugabe's government began seizing thousands of white-owned farms as part of a land redistribution policy designed to help poor blacks.

Many of the farms have ended up in the hands of senior officials with ZANU-PF and other supporters, while others have been tilled by farmers who lack experience and capital.

Zimbabwe now suffers chronic shortages of meat, milk, bread and other basic foodstuffs and relies on imports and handouts from foreign governments and relief agencies to feed its people.

Mugabe blames the country's economic collapse on sanctions imposed by foes in the West.

A senior Zimbabwean official said CARE and other suspended non-governmental agencies would only be allowed to pursue their work if they swore off involvement in politics.

"If they want to continue with their programmes, they know what to do. They must choose between politics and genuine humanitarian work," Deputy Information Minister Bright Matonga said, adding that Zimbabwe did not want to rely on NGOs.