On Thursday Amnesty International called him a prisoner of conscience and said the case is emblematic of a justice system rife with corruption, inefficiency and human rights abuse, despite efforts by President Vicente Fox to clean it up.

Especially in the countryside, local political bosses, or caciques, control the police and courts and use them to promote their own interests, silence opponents or exact revenge, said Rupert Knox, an investigator for the London-based rights group.

"It's a total cacique system," Knox said during a visit to Mexico. "One particular power group has a vested interest in maintaining their power."

The charges against Arriaga, 55, resemble earlier, high-profile cases in which environmental activists who blocked corporate logging operations were arrested and freed months or years later, Knox said.

In 2001, Fox freed Guerrero activists Rodolfo Montiel and Teodoro Cabrera under mounting international pressure. They have since taken their case to an international rights court to seek compensation and clear their names.

Arriaga's arrest might have been motivated in part as revenge over that case, Knox said.

He and other Amnesty investigators have traveled to Guerrero and other rural states during the past two weeks to document rights abuses.

Fox proposed legislation last year to overhaul the justice system. But opposition lawmakers in Congress have stripped the bill of key elements and it appears to be going nowhere.