President Robert Mugabe's regime struck at the opposition leadership Thursday only two weeks before Zimbabwe's presidential runoff election, twice detaining his challenger and jailing the No. 2 opposition official to face treason charges.

The U.S. ambassador, meanwhile, said that 20 tons of American food aid heading to impoverished Zimbabwean children had been seized by authorities last week and given to Mugabe supporters at a rally.

Morgan Tsvangirai, who led the opening round of presidential voting 2 1/2 months ago and faces the increasingly autocratic Mugabe in a June 27 runoff, was stopped at a roadblock in the south and held at a police station for about two hours, his party said.

The party said Tsvangirai went back to campaigning, but was stopped later by another group of police. It was the third and fourth times in recent weeks that he was detained while running against Mugabe, the longtime ruler increasingly unpopular for repressive ways and a wrecked economy.

But the biggest blow was aimed at Tendai Biti, secretary-general of Tsvangirai's Movement for Democratic Change, who was arrested at Harare airport upon returning from South Africa. Police said he would be charged with treason, which carries the possibility of the death penalty.

Police spokesman Wayne Bvudzijena said the treason charge related to a "transition document" discussing changing Zimbabwe's government.

He said Biti also would charged with making false statements "prejudicial to the state." That charge refers to accusations that Biti announced election results before the official count was released. Under Zimbabwean law, only the electoral commission can announce results.

Bvudzijena said Biti was in police custody but would not say where. He said Biti would be charged "as soon as we are through with our investigation," but would not be more specific.

Biti's detention robs the opposition of one of its most impassioned spokesmen. He has led on-and-off talks with Mugabe's party, and his arrest may signal Mugabe's final rejection of the possibility of negotiating Zimbabwe out of its political and economic crisis.

In a statement, Tsvangirai's party called on police "to immediately reveal Mr. Biti's location and release him unharmed immediately."

The party said it was "extremely concerned about the welfare of the secretary-general given the flagrant disregard for the rule of law and ongoing, state-sanctioned political violence and abductions currently prevalent in Zimbabwe."

Tsvangirai, Biti and other opposition leaders had left Zimbabwe soon after the first presidential ballot March 29 amid concerns about their security. Tsvangirai returned May 24 to begin campaigning for the runoff.

U.S. Ambassador James McGee said the Bush administration was "very, very concerned" about Biti's arrest.

McGee said he had seen the opposition party's "transition document" mentioned by the police spokesman, describing it as a routine plan that any political party would draw up to identify priorities if it were to come to power.

But he said a forged version had been circulating that raised issues not contained in the genuine document, including calls for punishing Mugabe hard-liners. "It was just a bunch of foolishness," he told The Associated Press.

McGee said continuing political violence, Biti's arrest and Tsvangirai's detention left him with little confidence that the runoff will be free and fair.

Still, he said, "I don't think we have any choice but to move forward with an election," saying that to do otherwise would be to hand victory to Mugabe.

McGee called on Zimbabwe's neighbors to intervene, saying the Southern African Development Community should send more observers to ensure peace before and during the vote.

Officials of the regional group said 120 monitors would deploy beginning Thursday and plans called for a total of 400 observers by election day — three times the number sent for the March 29 vote.

"We'd like to see three to four times that," McGee said. "Then I think we would have an opportunity" for free and fair elections.

The opposition, McGee and other foreign diplomats, and Zimbabwean and international human rights groups accuse Mugabe of unleashing violence against Tsvangirai's supporters to ensure Mugabe wins the runoff.

The government and Mugabe's party deny the allegations.

Before flying from Johannesburg to Harare, Biti said he had been informed he would be arrested. He said returning under the threat of arrest was "a stupid decision," but said he felt compelled to continue the battle for change. He spoke firmly, but trembled and sounded uncharacteristically discouraged.

"The only crime I have committed is fighting for democracy," he said at the Johannesburg airport, then hugged an aide and disappeared through the boarding gate.

Biti said efforts to negotiate a unity government had collapsed, calling it "sad." He said talks should be taking place instead of a runoff he predicted would only lead to more violence.

The opposition insisted that Tsvangirai be president of a coalition government and that Mugabe had no place in it. Mugabe's party demanded the longtime leader remain president.

The U.S. ambassador reported that a provincial governor confiscated a truck loaded with 20 tons of American wheat and beans intended for poor schoolchildren last week and ordered that the food be distributed to Mugabe supporters at a rally.

"This food assistance belongs to the U.S. government, to the U.S. taxpayer," McGee told The Associated Press, saying he had lodged a formal complaint with Zimbabwe's government Tuesday. He said he had not yet received a response.

"The bottom line is, they don't care," McGee said. "President Mugabe and his henchman are now looting U.S. government aid."

Mugabe, in power since the country gained independence from Britain in 1980, was lauded early in his rule for campaigning for racial reconciliation and building the economy. But in recent years, he has been accused of holding onto power through fraud and intimidation and trampling on people's rights.

He also is accused of overseeing an economic slide blamed on the collapse of the key agriculture sector after often violence seizures of farmland from whites.

Mugabe claimed he ordered the seizures, begun in 2002, to benefit poor blacks. But many of the farms went to his loyalists.