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

The U.S. ambassador, meanwhile, said 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.

The repeated detentions, coupled with Western accusations that Mugabe's regime is using food as a weapon, dramatically demonstrate the obstacles to the campaign thrown up by the longtime leader.

"This is a government that is taking tremendous and, frankly, awful strides to maintain its power, that is increasingly abusing its own citizens and has raised, or should I say lowered, the bar to a level that we rarely see," State Department spokesman Gonzalo Gallegos said in Washington.

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 first 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, and it was not known if he was still being held Thursday night. It was the third and fourth times in recent weeks that he was detained while running against Mugabe, who is 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

Imported from external blog