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.
A group of about a dozen supporters were blocked by uniformed and plainclothes police on 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 trial opened under heavy security on 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.
