HARARE (Sapa-AP) - The No. 2 opposition official will be charged with treason - which can carry the death penalty - police said Thursday, hours after Tendai Biti was arrested at the Harare airport upon returning from South Africa.

Meanwhile, Biti's boss, Morgan Tsvangirai - who faces off against increasingly autocratic President Robert Mugabe in a June 27 presidential runoff - was detained at a roadblock in southern Zimbabwe and taken to a police station, the party said. Two similar recent detentions have been brief.

Biti, secretary-general of Tsvangirai's Movement for Democratic Change party, also will be charged with making false statements "prejudicial to the state," police spokesman Wayne Bvudzijena said.

The treason charge relates to what Bvudzijena described as a transition document discussing changing Zimbabwe's government.

The second 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 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 the possibility of negotiating Zimbabwe out of its political and economic crisis.

Tsvangirai himself returned to Zimbabwe on May 24. He, Biti and other opposition leaders had left Zimbabwe soon after the first round March 29 amid concerns about their security.

U.S. Ambassador James McGee said his government was "very, very concerned" about Biti's arrest and word that he would be charged with treason. He said he had seen the MDC's transition document, describing 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 contained in the genuine document, including calls for punishing Mugabe hard-liners. "It was just a bunch of foolishness," McGee told The Associated Press.

McGee said continuing political violence, Biti's arrest and Tsvangirai's detention - his third in recent weeks - left him with little confidence that the runoff will 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 neighbours to intervene, saying the Southern African Development Community should send more observers to ensure peace before and during the vote.

SADC officials in Harare said they would deploy 400 observers, with an initial deployment of 120 starting Thursday. That was three times the number deployed for the March 29 vote.

"We'd like to see three to four times that," McGee said of the 400.

"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.

Zimbabwean government and party spokesmen deny the allegations.

Also Thursday, McGee said a provincial governor last week 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 said, adding that he has lodged a formal complaint but has not 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.

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 political and human rights.

He also is accused of overseeing an economic decline blamed on the collapse of the key agriculture sector after the seizures - often violent - 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.

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