By Agence France-Presse
The case against the pro-democracy activists, including high-profile lawyer Nguyen Van Dai, has garnered widespread attention in the one-party state, where a hardline leadership in place since 2016 is accused of cracking down on critics.
The accused are linked to the Brotherhood for Democracy, which bills itself as an activist network with about 80 full-time members across the country.
Independent civil society groups, political parties and media are all barred by the communist state in Vietnam.
A group of about a dozen supporters were blocked by uniformed and plainclothes police Thursday morning as they marched toward the courthouse in central Hanoi.
At least two were hauled into unmarked vans by plainclothes security agents and others put on a bus, according to an AFP reporter at the scene who was also questioned by authorities.
The group carried signs reading "Democracy is not a crime" and "Oppose suppression of Brotherhood for Democracy" before their march was broken up.
The trial opened under heavy security Thursday with the activists charged under Article 79 of the criminal code, which carries a maximum sentence of death.
They are accused of carrying out human rights training, calling for multi-party democracy and receiving funding from foreign groups, according to the indictment.
Nguyen Van Dai, 48, one of Brotherhood for Democracy's founders, was arrested along with his assistant Le Thu Ha in December 2015 following a human rights meeting with European Union officials in Hanoi.
They were initially charged with anti-state propaganda, later upgraded to the more serious charge of attempting to overthrow the state.
Dai's wife told AFP this week that his health has deteriorated during the 27 months of pre-trial detention.
"When I last met him, I think his spirit remained strong but he didn't look good... his hair has turned totally white," Vu Minh Khanh said, adding "he's done nothing wrong".
Dai has long been a thorn in the side of the communist authorities and served four years in prison for anti-state activity from 2007 to 2011.
The four others on trial were arrested in July 2017 and all have prior convictions as well.
Bloggers, lawyers and activists are routinely jailed in Vietnam and Amnesty International said this week there are 97 political prisoners currently behind bars in the country.
"Vietnam is one of Southeast Asia's most prolific jailers of peaceful activists -- a shameful title no one should aspire to," regional Amnesty director James Gomez said in a statement.