The trials began on Saturday of prominent moderates arrested shortly after Iran's disputed June presidential election and charged with fomenting unrest, Iranian media reported.
The semi-official Fars news agency says there are more than 100 defendants at the court, in what is Iran's first trial of people detained following the June 12 election.
The vote plunged Iran into its biggest internal crisis since the 1979 Islamic revolution and exposed deepening divisions in its ruling elite.
Moderate defeated presidential candidates Mirhossein Mousavi and Mehdi Karoubi say the vote was rigged in favor of hardline President Mahmoud Ahmadinejad.
The authorities deny the charge and Iran's Supreme Leader, Ayatollah Ali Khamenei, has endorsed Ahmadinejad's re-election.
"The trial of some of those accused of being involved in post-election unrest started this morning," Fars news agency reported.
Those on trial included prominent reformers such as former vice president Mohammad Ali Abtahi, former Deputy Foreign Minister Mohsen Aminzadeh, former government spokesman Abdollah Ramazanzadeh, former senior lawmaker Mohsen Mirdamadi and former Industry Minister Behzad Nabavi, Fars reported.
Iranian media have reported that some of those detained were accused of acting against national security, a common charge against dissenting voices in Iran. They could even face the death penalty.
On Thursday thousands of pro-reform demonstrators gathered to mourn those killed in post-election unrest, in a show of defiance of the authorities upholding Ahmadinejad's victory. Riot police fired tear gas and arrested some protesters.
A police official told the semi-official ILNA news agency on Friday that 50 protesters had been arrested but said "many of them have been released later".
Rights groups say hundreds of people, including senior pro- reform politicians, journalists, activists and lawyers, have been detained in Iran since the election.
Iran's top judge, Ayatollah Mahmoud Hashemi-Shahroudi, ordered the judiciary on Monday to follow the cases of detained protesters.
Meanwhile, Iran's state owned Arabic-language al-Alam TV station confirmed that three US citizens were arrested after crossing the border from Iraq.
The report cited a "well-informed source" in the Interior Ministry that the three Americans were detained Friday after crossing into Iran's Kurdistan province.
The report said the Americans were arrested after they refused to heed warnings from border guards.
AP contributed to this report