Iran's biggest reformist party has dismissed the court appearance of 100 people, including leading opposition figures, as a "laughable show trial", BBC reported.
The accused are on trial for alleged involvement in post-election violence, on charges including acting against national security and vandalism.
Pro-government media reported what they said were confessions by some of the leading reformists.
But the party, Mosharekat, said the "confessions" had been forced.
It said "even a cooked chicken" would laugh at the charges.
The party was the principal backer of Mir Hossein Mousavi, the main opposition candidate in the 12 June presidential elections.
Meanwhile on his website Mr Mousavi rejected the authorities' claims that Western countries had fuelled the post-election unrest.
'National heroes'
Kasra Naji, special correspondent for BBC Persian Television, says the timing and scale of the trial came as a surprise and suggests Iran's leadership wants to send a message to stop any more protests.
But judging from messages on micro-blogging site twitter and the internet, our correspondent says, the move may have the opposite effect, with several people talking about the need for new demonstrations and calling those on trial "national heroes".
Some of the defendants told the court their earlier claims of fraud during the 12 June poll were baseless, official media said.
Allegations of vote-rigging were made by defeated candidates and their supporters as soon as it became clear President Mahmoud Ahmadinejad had been re-elected by a large margin.
But former vice-president Mohammad Ali Abtahi was quoted by Fars news agency as telling the court: "I say to all my friends and all friends who hear us, that the issue of fraud in Iran was a lie and was brought up to create riots."
He said that the aim was to create a "velvet revolution", referring to the overthrow of Communism in Czechoslovakia in 1989.
Restrictions
At the trial, pictures from the packed courtroom showed seated defendants wearing prison uniforms and with guards next to them.
Foreign media, including the BBC, have been restricted in their coverage of Iran since the election protests turned violent.
Official news agency Irna said other charges against the accused included "having ties with counter-revolutionary groups", rioting and conspiring against the ruling system.
The defendants included supporters of opposition leaders Mr Mousavi and Mehdi Karroubi - both defeated in the election - and aides of former reformist president Mohammad Khatami.
Fars news agency reported that former deputy foreign minister Mohsen Aminzadeh, former government spokesman Abdollah Ramazanzadeh, former senior lawmaker Mohsen Mirdamadi and former Industry Minister Behzad Nabavi were among the defendants.
The protests were the largest mass demonstrations seen in Iran since the 1979 revolution, which brought the current Islamic regime to power.
In the days of violence following the re-election of President Ahmedinejad at least 30 people were killed.
Authorities also arrested hundreds during the protests.
About 140 people arrested at the time were released from prison on Tuesday, with a further 200 accused of more serious crimes remaining in prison.
Opposition groups believe the number of prisoners and those killed in the violence to be higher.
Mr Ahmadinejad is due to be officially sworn in on 5 August.
Clashes have continued since his election, most recently during mourning to mark 40 days since the death of Neda Agha Soltan, who was shot as she watched protests on 20 June.