The trial of leading human rights lawyer Le Cong Dinh and three other democracy activists for subversion has begun in Vietnam.
The four are accused of plotting to overthrow the communist government.
The subversion charge carries a minimum sentence of 12 years and a maximum sentence of execution.
The case has drawn strong criticism from rights groups abroad, who see it as a sign of an increasing clampdown on democracy and freedom of expression.
The hearing is scheduled to last two days.
Activists' defender
It has been a long time since the communist regime tried anybody on subversion charges, considered one of the most serious offences under Vietnam's criminal code, says the BBC's Nga Pham.
The four accused - who were arrested in June - were initially charged with spreading anti-government propaganda.
But early last month, state prosecutors decided to bring more serious charges.
While execution is unlikely, the dissidents are still facing years in prison for their pro-democracy activities if they are found guilty, our correspondent says.
This trial is seen by many as a showcase of the government's intolerance against political dissent in the run-up to next year's Communist Party Congress.
Mr Dinh rose to prominence when he represented Vietnam's interests in the so-called "catfish battle" where US farmers accused the Vietnamese of dumping cheap seafood on the US market.
In recent years, Mr Dinh defended some of Vietnam's leading human rights and democracy activists, such as fellow lawyers Nguyen Van Dai and Le Thi Cong Nhan, who have also been jailed for anti-government activity.
