The trials have begun of 100 prominent moderates arrested shortly after Iran's disputed June presidential election and charged with trying to overthrow the clerical establishment.

This is the first time since Iran's 1979 Islamic revolution that dozens of senior officials, including former ministers, vice-presidents and lawmakers, have been put on trial.

The official IRNA news said the charges against the defendants also included acting against national security by planning unrest, participating in the "Velvet Revolution", attacking military and state buildings and conspiring against the ruling system.

"Some 100 people have been put on trial in a Tehran Revolutionary court," IRNA said.

Velvet Revolution was used to describe the non-violent 1989 revolution in Czechoslovakia which overturned communist rule.

Under Iran's Islamic law, acting against national security, a common charge against dissenting voices in Iran, could be punishable by the death penalty.

The 12 June vote plunged Iran into its biggest internal crisis since the 1979 Islamic revolution and exposed deep divisions in its ruling elite.

Rights groups say hundreds of people – including senior pro-reform politicians, journalists and lawyers – have been detained since the election.

State television showed footage of the courtroom with many young defendants, some handcuffed, and vice-president Mohammad Ali Abtahi, former deputy foreign minister Mohsen Aminzadeh and former MP Mohsen Mirdamadi in prison uniform.

On trial also are prominent members of Iran's leading moderate parties, founded by former presidents Akbar Hashemi Rafsanjani and Mohammad Khatami. Both are backers of moderate defeated candidate Mirhossein Mousavi.

The indictment said: "These parties planned, organised and led the illegal gatherings and riots. The Participation Front had contacts with a British spy." The Participation Front is the main pro-reform party, set up by Khatami.

Iran accuses western countries, particularly Britain and the United States, of supporting "rioters". The west denies it.

Leading moderates say the vote was rigged in favour of hardline President Mahmoud Ahmadinejad.

The authorities deny the charge and Iran's Supreme Leader, Ayatollah Ali Khamenei, has endorsed Ahmadinejad's re-election.

Hardline semi-official Fars news agency said at least four prominent reformers now said that the vote was not rigged.

"Former vice-presidents Mohammad Ali Abtahi and Mohsen Safai-Farahani, former industries minister Behzad Nabavi, [Iranian-Canadian journalist] Maziar Bahari and former deputy interior minister Mostafa Tajzadeh have confessed that the issue of fraud in the Iran vote was baseless," Fars reported.

Bahari, who came to Iran to cover the vote for Newsweek, has said that he took cash from Britain's Channel 4 television for sending footage of unrest, Fars said, adding he had told reporters that foreign media were involved in the unrest.

Fars also said detained Iranian photographers Majid Saeedi, who worked for Getty Images, and Satiar Emami had said they sold pictures of "riots" to British and French media.

IRNA said Kian Tajbakhsh, a US citizen who in 2007 was accused of spying and detained for four months, was among those who were tried for being involved in the unrest. Tajbakhsh was detained in early July in Tehran.

"The post-election developments were planned from a year ago by Americans," Tajbakhsh told IRNA after the trial.

Last Tuesday, Iran released 140 detained protesters with "minor charges" from Tehran's Evin prison, but 250 others remained in jail.