Days of peaceful protest in Yerevan turned violent in March
A former foreign minister of Armenia and six opposition politicians are being tried for seeking to overthrow the government earlier this year.
Alexander Arzoumanian is also accused of organising protests in March against election results, in which 10 people were killed.
The opposition says the trial is politically motivated. He and his co-accused deny the charges.
More than 50 people have already been convicted for being part of the unrest.
Opposition supporters cheered the defendants as they entered the courtroom with cries of "We are with you!" and "Free political prisoners!"
The accused face up to 15 years in prison if convicted of seeking to "usurp state power".
Chief investigator Vahagn Harutyunyan said the case was based on evidence from 500 civilian and police witnesses.
"We have records of telephone conversations, private video recordings and television footage, and public speeches by opposition representatives," he told the Reuters news agency.
Mr Arzoumanian, who was foreign minister in the 1990s, is on trial alongside six others, including three members of parliament.
Earlier this year, he was campaign manager for former Armenian President Levon Ter-Petrosian, who finished second in the poll.
The opposition said the vote, won by Serzh Sarkisian, was rigged.
The unrest arose when thousands Mr Ter-Petrosian's supporters rallied in the Armenian capital, Yerevan, for 11 days, before street battles broke out with riot police.
Two police officers and eight civilians were killed.
A state of emergency was declared and army vehicles were deployed on the streets before the protests were eventually dispersed.