A court in southern Vietnam began a trial on Monday against 22 people accused of trying to overthrow the government, a court official said.
The activists are accused of being members of the Hoi Dong Cong Luat Cong An Bia Son political organisation, local news reported.
According to the indictment, the group was disguised as a tourist company operating out of a resort in Phu Yen province. The defendants are also accused of receiving funds from Vietnamese living in the United States.
If found guilty they could face the death penalty. The court is scheduled to announce a verdict in five days.
Vietnam has increased its crackdown on dissidents in recent years despite criticism from western governments.
Earlier this month, 14 activists received sentences of up to 13 years.