Opposition supporters have denounced the sessions as "show trials"
The trial has begun in Iran of a number of senior opposition figures following June's disputed presidential election.
The defendants, who include former ministers in the 1997-2005 Khatami government, are accused of conspiring with foreign powers to organise unrest.
One leading reformist, Saeed Hajjarian, apologised for his "grave mistakes".
It is the fourth such trial since the disputed re-election of President Mahmoud Ahmadinejad, a conservative, sparked pro-reform street protests.
BBC Tehran correspondent Jon Leyne - who was expelled after the elections - says the trial looks like a public denunciation of former President Mohammed Khatami's time in power, with the government trying to frighten the opposition into silence.
12 June presidential election saw incumbent Mahmoud Ahmadinejad re-elected with 63% of vote
Main challenger Mir Hossein Mousavi called for result to be annulled, alleging poll fraud
Mass street protests saw at least 30 people killed and foreign media restricted
Hardliners are currently also pressing for the arrest of the two leading opposition candidates in the election, Mir Hossein Mousavi and Mehdi Karroubi.
The court proceedings are open only to Iranian news agencies and have been denounced as "show trials" by opposition leaders.
The 20 people in the dock on Tuesday included former Deputy Interior Minister Mostafa Tajzadeh, former Deputy Foreign Minister Mohsen Aminzadeh and former government spokesman Abdollah Ramezanzadeh, reports said.
Also on trial is leading economist Saeed Leylaz and Kian Tajbaksh, an Iranian-American.
Mr Hajjarian, a former deputy minister turned architect of the reformist movement under former President Khatami, was also in court.
A prosecutor read out a long list of charges against Mr Hajjarian, including acting against national security, casting doubt on the election result, propagating against the regime and insulting regime officials, including Supreme Leader Ayatollah Ali Khamenei.
He called for the "maximum sentence" against Mr Hajjarian, though it is not clear what this would be.
Partially paralysed after he survived an assassination attempt in 2000, Mr Hajjarian has difficulty speaking and after he identified himself a statement was read out for him by another defendant.
In the statement, Mr Hajjarian apologised for his "grave mistakes by offering incorrect analysis during the election", reported Iranian state news agency Irna.
He said he was resigning from the Islamic Iran Participation Front - the main opposition party, also known as Mosharekat - and announced his "complete adherence to the constitution and... to the Supreme Leader".
Many defendants in the previous three rounds of trials have also confessed, though critics say some were put under duress. No sentences have yet been announced.
Opposition groups alleged widespread vote-rigging in the June election, of which Mr Ahmadinejad was declared the winner by a landslide.
It led to the largest mass demonstrations in Iran since the 1979 revolution, which brought to power the current Islamic system of government.