The trial underlines the government's efforts to bring to a close anti-government demonstrations that have persisted since the disputed June 12 presidential election.
Hundreds of thousands of Iranians marched in days of street protests after the election, denouncing official results that declared President Mahmoud Ahmadinejad the winner.
Iran's opposition maintains Ahmadinejad stole the vote from opposition leader Mir Hossein Mousavi by engaging in massive fraud, but its demonstrations have been ruthlessly suppressed, leaving hundreds in prison.
The defendants faced charges that include attacking military and government buildings, having links with armed opposition groups and conspiring against the ruling system, Iran's official news agency, IRNA, reported.
During the session, prosecutors read out an indictment outlining what they said was a yearslong plot by the top pro-reform political parties to carry out a "velvet revolution," a popular, non-violent uprising to overthrow the Islamic Republic similar to ones that have occurred in Eastern Europe.
The phrase comes from the peaceful 1989 Velvet Revolution that overthrew decades of communism in Czechoslovakia.
The prosecutor said three of the biggest opposition parties had taken money from foreign non-governmental organizations and had sought to use the election controversy as an opportunity to carry out their plot, according to a transcript reported by IRNA. He claimed Israeli and Western officials have spoken in recent years of fomenting revolution in Iran.
"Based on the evidence obtained and well-founded confessions of the defendants, these events had been planned in advance and stages of the velvet revolution were carried out in accordance with a time schedule," the 15-page text of the indictment said.
IRNA did not give information about how many defendants were in court, but the semiofficial Fars news agency said more than 100 defendants were present.
Among the defendants were several prominent reformist opposition activists, including former Vice President Mohammad Ali Abtahi, former government spokesman Abdollah Ramezanzadeh, former Vice Speaker of parliament Behzad Nabavi, former Deputy Foreign Minister Mohsen Aminzadeh and leader of the biggest reformist party, the Islamic Iran Participation Front, Mohsen Mirdamadi.
The reformist Web site http://www.mowjcamp.com denounced Saturday's trial and said defendants had no access to lawyers and there was no jury.
"Do those who organized this show trial today think that the nation will remain silent to slaughter the nation's best?