JOHANNESBURG - With only three weeks to go before Zimbabwe's presidential runoff, the police briefly detained the opposition's standard-bearer, Morgan Tsvangirai, on Friday for the second time this week and directed his party to cancel political rallies, effectively preventing him from addressing voters.

Morgan Tsvangirai, the opposition presidential candidate, campaigning Friday. The police, citing concerns about safety for his party's leaders, ordered the cancellation of rallies he had planned.

At the same time, the Zimbabwean government's requirement that all nongovernmental organizations suspend their aid operations, which grew out of the authorities' allegations that some were supporting the opposition, was condemned Friday by officials in the United States and Europe, as well as the United Nations.

Relief agencies said the order, issued this week, would deprive millions of desperately poor Zimbabweans of food aid and other basic assistance. Unicef, for example, depends on 25 nongovernmental organizations to provide education, health care and food to 185,000 orphans in Zimbabwe.

"It's a horrible situation," said James Elder, Unicef's spokesman in Zimbabwe. "The children and their families continue to find stoic means of surviving, but this is a profoundly disturbing blow to them. We can't reach these children today."

Similarly, the World Food Program said on Friday that the prohibition on aid operations would prevent "314,000 of the most vulnerable people in the country" - the elderly, the disabled, schoolchildren, tuberculosis patients and "H.I.V.-positive bedridden" people - from receiving food this month.

World Vision, another large relief agency, said it had planned to feed about 400,000 people in Zimbabwe in June and was particularly concerned about the welfare of the 1.3 million children under 5 who had been orphaned by AIDS.

President Robert Mugabe has led Zimbabwe, which has a population of about 13 million, for almost three decades. In the past few years, the country's economy has gone into free fall, with more than four in five people unemployed and prices of food staples sent into the stratosphere by hyperinflation. The space for peaceful political protest keeps shrinking, according to officials of Mr. Tsvangirai's party, the Movement for Democratic Change.

The party said it had received a written notice from the police that rallies it had planned in impoverished townships of the capital, Harare, would have to be canceled because the safety of party leaders could not be guaranteed - a seemingly paradoxical rationale, given that the police had confiscated Mr. Tsvangirai's armored vehicle on Wednesday.

The government's decision to block rallies is a blow to Mr. Tsvangirai, a charismatic figure who drew large, enthusiastic crowds before the March election.

He has survived three assassination attempts and was severely beaten by the police in March 2007. In the voting 10 weeks ago, he finished first, but according to official returns he did not have the majority he needed to avoid a runoff election against Mr. Mugabe.

Mr. Tsvangirai left the country not long after that election, fearing another assassination attempt. He returned to Zimbabwe two weeks ago, but he has since been unable to campaign freely for the June 27 runoff vote.

Opposition officials said Mr. Tsvangirai was detained on Wednesday for nine hours and again on Friday for two hours. He was stopped at a roadblock on his way to a rally not far from Bulawayo, Zimbabwe's second largest city, and sent to a police station for questioning.

A police spokesman, Wayne Bvudzijena, blamed the opposition for Mr. Tsvangirai's most recent detention, alleging that the candidate's convoy crashed through a roadblock, according to Reuters.

Zimbabwean police officers and soldiers detained a contingent of American diplomats for five hours on Thursday at a roadblock, slashing the tires of their vehicle after a six-mile chase. The diplomats had been investigating state-sponsored violence against the opposition. The Bush administration has expressed outrage at the police action. The State Department said it would seek a discussion by the United Nations Security Council of the mistreatment of its diplomats.

The Zimbabwean authorities have been aggressively using the state's power to detain and arrest many of those whom they regard as a threat to the governing party's hold on power. They have also been forcefully exerting the state's monopolistic control over television, radio and the nation's only daily newspaper.

This week, eight employees of the Zimbabwe Broadcasting Corporation were sent on leave for two months - a step that the Media Institute of Southern Africa said "can reasonably be perceived as the deliberate purging of journalists at the state broadcaster for the purposes of partisan political expediency."

The state news media's coverage of Mr. Tsvangirai, a former trade union leader who has led the opposition for almost a decade, is deeply hostile. He is typically depicted as a coward, a fool and a stooge of Britain, the former colonial power. Zimbabweans have a powerful informal grapevine, spurred by the technology of the text message, but rallies were Mr. Tsvangirai's principal means of communicating directly with voters.

To tell a man like Mr. Tsvangirai he cannot speak at rallies is "like telling a pastor not to read the Bible," said an opposition spokesman, Nelson Chamisa, who added, "That's depriving a politician of the oxygen that helps keep a political institution alive and kicking."