Police briefly detained Zimbabwe's opposition presidential candidate Friday for the second time this week and told him the party's rallies had been banned indefinitely three weeks before the runoff election, an aide said.

The latest setback for Morgan Tsvangirai's campaign came as the U.S. ambassador to Zimbabwe accused President Robert Mugabe's regime of using food as a weapon to stay in power and warned "massive, massive starvation" could result.

Aid groups in Zimbabwe were ordered Thursday to halt operations in a move that could hamper food deliveries. Without the private aid groups, impoverished Zimbabweans will be dependent on the government and Mugabe's party.

U.S. Ambassador James McGee told reporters Friday by video conference that the regime is distributing food mostly to its supporters and that opposition supporters are offered food only if they hand in identification that would allow them to vote.

In a statement released Friday, Tsvangirai's Movement for Democratic Change said police had banned the party's rallies out of concern for the safety of Tsvangirai and other party leaders. The open-ended ban only affects the opposition.

Sibotshiwe called the justification "nonsense," and said the ban was "a clear indication that the regime will do everything necessary to remain in power."

Tsvangirai beat Mugabe in the March 29 first round, but did not garner the 50 percent plus one vote necessary to avoid a runoff, which is scheduled for June 27.

Opposition and human rights groups accuse Mugabe of orchestrating violence to ensure he wins re-election amid growing unpopularity for his heavy-handed rule and the country's economic collapse.

Tsvangirai had been trying to campaign Friday around Bulawayo, Zimbabwe's second largest city. He was stopped at two roadblocks, and the second time was ordered to a police station about 30 miles from Bulawayo.

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

Tsvangirai was questioned by police for 25 minutes at the station, and was told that all party rallies in the country had been banned indefinitely, Sibotshiwe said.

On Wednesday, Tsvangirai said he was detained for nine hours at another police station near Bulawayo. Police spokesman Wayne Bvudzijena denied police were interfering with the opposition campaign.

Bvudzijena said he was not aware of Friday's incidents, but said that it is not uncommon for police to stop drivers at roadblocks to ensure they are not transporting weapons.

"Tsvangirai and his convoy are not immune to search," he said. "They can be searched at any roadblock they pass."

He also said candidates had been told they needed to inform police before holding a political rally.

On Thursday, a mob of Zimbabwe "war veterans," a group of often violent Mugabe loyalists, waylaid a convoy of American 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.

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

Millions of Zimbabweans depend on international groups for food and other aid as the economy crumbles. The world's highest inflation rate has put staples out of reach in what was once the region's breadbasket.

Jendayi Frazer, the assistant U.S. secretary of state for African affairs, appealed to South Africa's president to put pressure on Mugabe "not to starve the population and to allow international organizations to function."

"It's unbelievable that the government will actually kick out the organizations which are providing services to the people," Frazer said of the suspension of aid organizations' operations.

South African President Thabo Mbeki, who is the main mediator in Zimbabwe, made no reference to the crisis Friday at the World Economic Forum on Africa in Cape Town.

In London, British Development Aid Secretary Douglas Alexander said the decision was evidence of a callous contempt for human life.

James Elder, a spokesman for the UN children's agency, said the suspension was "completely unacceptable and hugely concerning. Hundreds of thousands of children are in need of immediate assistance.

"With the onset of the winter in Zimbabwe, the timing is critical for children who are among the most vulnerable and most in need of support," Elder said.

Goche's memorandum to the United Nations and other aid groups made no mention of 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's party.

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.

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 beginning in 2000.

Discontent over the economy propelled Tsvangirai to the top in presidential voting March 29.

Tsvangirai, who lost a 2002 presidential election that independent observers said was rigged in Mugabe's favor, had only returned to Zimbabwe in late May to campaign for the runoff. He left the country soon after the March first round, and his party has said he was the target of a military assassination plot.

He has survived at least three assassination attempts. In 1997, unidentified assailants tried to throw him from a 10th-floor window.

Last year, he was hospitalized after a brutal assault by police at a prayer rally. Images seen around the world of his bruised and swollen face have come to symbolize the plight of dissenters in Zimbabwe.

Tsvangirai's Movement for Democratic Change says at least 60 of its supporters have been slain in the past two months.