HARARE, Zimbabwe — Police said Zimbabwe's No. 2 opposition official would be charged with treason, a potential death penalty charge that marked a dramatic escalation of a government crackdown ahead of a presidential runoff.

Tendai Biti, the Movement for Democratic Change's secretary-general, was arrested Thursday at the Harare airport upon returning from South Africa, party spokesman Nqobizitha Mlilo said. The party said he had been taken to an unknown location.

The treason charge relates to a "transition document" discussing changing Zimbabwe's government, police spokesman Wayne Bvudzijena said.

Bvudzijena said that Biti would also be charged with making false statements "prejudicial to the state," a charge that refers to accusations 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 formally charged "as soon as we are through with our investigation," but would not be more specific.

Party officials said separately that MDC leader Morgan Tsvangirai, — who faces longtime leader Robert Mugabe in the June 27 presidential runoff — was detained again at a roadblock and taken to a police station. He was released soon after and resumed campaigning, the party said.

It was the third time in recent weeks Tsvangirai has been briefly detained while trying to campaign against the increasingly autocratic Mugabe.

U.S. Ambassador James McGee said Washington was "very, very concerned" about Biti's arrest and word he would be charged with treason. McGee said he had seen the Movement for Democratic Change's transition document, and described it as a routine plan any political party would draw up to identify priorities if it were to come to power. But he said a forged version had circulated that raised issues not in the genuine document, including calls for punishing Mugabe hardliners.

"It was just a bunch of foolishness," McGee said.

The MDC said in its statement that plainclothes police arrested Biti just after he got off the plane and before immigration. The party said 10 men then took him away in a truck.

Returning under threat of arrest was "a stupid decision," Biti said in Johannesburg, but added that he believed he must return to continue the battle for change. He spoke firmly, but trembled and sounded uncharacteristically discouraged.

He said he had been informed that he would be arrested but that it was not clear on what charges.

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

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

Tsvangirai had himself only returned to Zimbabwe on May 24. He, Biti and other opposition leaders left Zimbabwe soon after the first round, amid concerns about their security, to lobby support among southern African regional leaders.

Tsvangirai came in first among a field of four March 29. His campaign has been beset by violence. The opposition, foreign diplomats in Zimbabwe, and Zimbabwean and international human rights groups accuse Mugabe of unleashing attacks against Tsvangirai's supporters to ensure Mugabe wins the runoff.

Zimbabwean government and party spokesmen repeatedly have denied such allegations.

But police also have stopped several opposition attempts to hold rallies since Tsvangirai's return. And the state-controlled media has all but ignored him in a country where few have access to the Internet or satellite television.

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

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

McGee called on Zimbabwe's neighbors to intervene.

He also said that last week, a Zimbabwean provincial governor confiscated a truck loaded with 20 tons of U.S. food aid for poor schoolchildren and ordered that its wheat and beans 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 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."

The incident occurred as aid agencies in Zimbabwe received word that the government had ordered them to suspend field work. That sparked accusations Mugabe is using food as a political weapon in a country where economic collapse has left many unable to afford groceries.

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Associated Press writer Donna Bryson contributed to this report from Johannesburg, South Africa.