Tsvangirayi Mukwazhi, Associated Press
Opposition leader Morgan Tsvangirai talks to people on campaign trail in Esigodini, Zimbabwe, on Friday. While campaigning, he was stopped at roadblocks and taken in for police questioning.

HARARE, Zimbabwe — Just three weeks before Zimbabwe's presidential runoff, Robert Mugabe is giving the opposition little room to campaign — detaining its candidate, banning rallies and attacking diplomats who try to investigate political violence.

Even food is being used as a weapon, American and British officials said, with a ban on aid agencies ensuring that the poorest Zimbabweans must turn to Mugabe for help even if they blame him for the collapse of the economy. The government denied the allegations.

Opposition leader Morgan Tsvangirai out-polled Mugabe and two other candidates in the first round of voting March 29 but did not get the simple majority necessary to avoid a runoff. In recent days, it has become increasingly clear that Mugabe does not plan to let Tsvangirai come close to toppling him in the June 27 runoff.

Tsvangirai tried to campaign around Bulawayo, Zimbabwe's second-largest city, on Friday, but he was stopped at two roadblocks. At the second, he was ordered to go to a police station about 30 miles from Bulawayo.

About two hours later, he and reporters with him were allowed to leave the station. They drove back to Bulawayo under police escort.

His spokesman, George Sibotshiwe, said Tsvangirai was questioned by police for 25 minutes and was told that all party rallies in Zimbabwe had been banned indefinitely.

"We are dismayed that our president has not been allowed to access the Zimbabwean people at a crucial stage in this campaign," Sibotshiwe said.

Tsvangirai's Movement for Democratic Change said police had banned its rallies out of concern for the safety of Tsvangirai and other party leaders. Sibotshiwe called the justification "nonsense" and said the ban was "a clear indication that the regime will do everything necessary to remain in power."

Police spokesman Wayne Bvudzijena said in an interview with The Associated Press that "people are free to campaign as they choose," but he said Tsvangirai had consistently broken the law by failing to notify police of his rallies.

"For now, we are just warning him," Bvudzijena said, "but sooner or later he might end up being arrested."

Tsvangirai left the country soon after the first round of voting, and his party has said he was the target of a military assassination plot. He has survived at least three previous attempts on his life. Tsvangirai returned to Zimbabwe in late May to campaign for the runoff.

The government-controlled media has focused on Mugabe and ZANU-PF, all but ignoring Tsvangirai's campaign, raising the question of whether Zimbabweans in isolated rural areas even know the opposition leader has returned.

Tsvangirai's party, blaming state agents, says at least 60 of its supporters have been killed in the past two months.

The latest setback for Tsvangirai came as U.N. aid agencies said they were deeply concerned that Zimbabwe has ordered aid groups to halt operations. Millions of Zimbabweans depend on international groups for food, medicine and other aid as the economy crumbles. Without the private agencies, impoverished Zimbabweans will be dependent on the government and Mugabe's party, both of which distribute food and other aid.

U.S. Ambassador James McGee said Zimbabwean authorities were now supplying food mostly to Mugabe supporters. In a videoconference to reporters in Washington from Harare, McGee said the U.S. Embassy has evidence that the government is offering food to opposition members only if they turn in identification that would allow them to vote.

State Department spokesman Sean McCormack referred to the tactics that McGee described as "a vicious attempt to use food as a political weapon."

At the United Nations, Zimbabwe Ambassador Boniface Chidyauskiku denied those charges.

"There is no use of food as a political weapon. It is the other way around. It is the relief agencies, followed by the U.S. government, that have been using food as a political weapon," Chidyauskiku told the AP.

"They have gone out into the countryside and they have been telling Zimbabweans that if you don't vote for the opposition, if you don't change your vote, there's no food for you," he said. "So it is the United States using food as a political weapon to effect a regime change in Zimbabwe. This is why we have suspended the activity."

On Thursday, aid groups in Zimbabwe were sent a memorandum from social welfare minister Nicholas Goche ordering an indefinite suspension of field work.

Aid deliveries to more than 4 million people in the country will be severely hampered by the decision, said Elisabeth Byrs, a spokeswoman for the U.N. Office for the Coordination of Humanitarian Affairs. More than half of Zimbabwe's population lives on less than $1 a day and life expectancy is only 35 years, according to the U.N.

U.N. agencies generally carry out their operations in the country with the help of other aid groups, Byrs said.

"These restrictions are also coming at a time when food security in Zimbabwe is deteriorating, leaving an increasing number of people vulnerable," she said.

Poor rain recently has increased the risk of drought, and farmers lack seeds, Byrs said.

Goche's memorandum to the United Nations and other aid groups did not mention government claims that aid was distributed to favored recipients or opposition supporters, or that civic and human rights groups registered as voluntary organizations were campaigning against Mugabe.

Earlier this week, the aid organization CARE International said it had been ordered to halt operations pending an investigation of allegations it was campaigning for the opposition. CARE denies the allegation.

Byrs said the suspension of CARE's activities alone would immediately affect half a million Zimbabweans.

On Thursday, a group of often violent Mugabe loyalists waylaid a convoy of U.S. and British diplomats investigating political violence, beating a local staffer, slashing tires and threatening to burn the envoys, the U.S. Embassy said.

Mugabe frequently accuses Britain and the United States of plotting to topple him and return Zimbabwe to colonial rule.

Mugabe has led Zimbabwe since independence from Britain in 1980 and was once hailed as a liberator who promoted racial reconciliation and economic empowerment.

But he has been accused of clinging to power through election fraud and intimidation, and of destroying his country's economy through the seizure of white-owned farms.

Carolyn Norris, an Africa specialist at Human Rights Watch, called the move against aid groups part of an "extreme campaign of violence and torture" for people who voted for Tsvangirai's party.

"We don't know if this will convince people to vote" for Mugabe, Norris said, adding Zimbabwe's population "seems determined to vote how it wants to."

Tsvangirai also said he expected Mugabe's crackdown to backfire, saying Thursday: "If Mugabe did not hear the voice in March, he's going to hear a much louder voice that people no longer enjoy their confidence in this government."