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 years-long 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 Green Freedom Wave (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?
Pictures from the courtroom showed a thin-looking Abtahi and a grim Mirdamadi, both in prison uniforms, sitting in the front row. More than a hundred defendants could be seen sitting in the packed courtroom, many of them handcuffed but without prison uniforms.
A reformist lawyer, Mohammad Reza Tabesh, quoted Abtahi's wife as saying that the former vice president had lost 40 pounds, or 18 kilograms, of weight after 43 days in custody.
There was no information on when the trial would end or when a verdict could be expected.
The post-election protests have marked the biggest challenge to the cleric-led regime's power since the 1979 revolution.
Ahmadinejad on Friday sought shelter from his top supporter, the country's supreme leader, Ayatollah Ali Khamenei, declaring that Khamenei is like a father to him. Ahmadinejad accused his hard-line rivals of trying to drive a wedge between him and the man who sits at the top of Iran's clerical leadership and who has final say in all state matters.
On Monday, Khamenei will lead a ceremony formally approving Ahmadinejad's second term, and two days later Ahmadinejad is to be sworn in before parliament. But recently Ahmadinejad has been beset not just by protesters attacking the election's legitimacy but also by rivals within his own hard-line camp.
Meanwhile, the anti-regime protests have continued with thousands of protesters holding a memorial at a Tehran cemetery on Thursday to commemorate those killed in the crackdown. Police fired tear gas and beat protesters with batons, but the march continued, as protesters chanted Mousavi's name.
Protesters then streamed back into central Tehran - some chanting on the subway, "Traitor Mahmoud, we want you to become homeless" - and again clashed with security forces.
By Associated Press Writer Ali Akbar Dareini