The government has come under pressure over its handling of protests
Authorities in Iran say 4,000 people were detained in protests that followed June's disputed presidential election - many more than previously stated.
A spokesman for the judiciary, Ali Reza Jamshidi, said about 3,700 had been freed within a week, but that those involved in riots were in detention.
An opposition figure has also claimed at least 69 people were killed in the violence following the 12 June poll.
It is more than double the number the government admits died.
Iran is currently trying more than 100 detainees over their alleged involvement in the protests.
The trials - of leading opposition figures, activists, journalists, lawyers, workers at foreign embassies and two people with foreign nationalities - have been criticised by several foreign powers, opposition groups and human rights campaigners.
But authorities insist their legal proceedings are completely legitimate and conform to international standards of justice.
Speaking at a news conference, judiciary spokesman Mr Jamshidi admitted some 4,000 people were detained in June's post-election street protests.
But he said only 300 - who had been "involved in the riots" - were held for longer than a few days.
Supporters of the protesters had previously challenged government figures on the numbers of those detained, suggesting it could be in the thousands.
The figure for those killed when June's street protests turned bloody - in violence the government blamed on "thugs" but which protesters blamed on security services - has also never been confirmed.
The police put it at about 20, but official figures later suggested about 30 had died.
Now Alireza Hosseini Beheshti - an ally of defeated election candidate Mir Hossein Mousavi - has said testimony from bereaved families suggests at least 69 people were killed.
He told AP news agency that the number was still rising and included victims from the capital, Tehran, and the rest of the country.
Offer to France
Among those on trial over the protests are an Iranian who worked at the UK embassy in Tehran, a French-Iranian who worked at the French embassy, and a French national - charged with crimes including spying and plotting to overthrow the government.
Frenchwoman Clotilde Reiss is among those being tried
On Tuesday, French embassy staffer Nazak Afshar was released from the Tehran prison where she was being held, but remains on trial, French President Nicolas Sarkozy announced.
Mr Sarkozy thanked the EU and "other friendly states, such as Syria" for help in her release.
He called for Clotilde Reiss, a French university teaching assistant who is also in prison, to be released without delay.
Iran offered to allow her to live in the French embassy in Tehran for the duration of the trial, French news agency AFP quoted Iran's ambassador in Paris as saying.
There had been no reported response from French authorities, the ambassador was quoted by AFP as saying.
But the French foreign ministry later dismissed his claim, saying Iran had been notified weeks ago that the French embassy was willing to take in Ms Reiss, according to AFP.
France has denounced the proceedings against Ms Reiss, but the action was defended by Hassan Qashqavi, a spokesman for Iran's foreign ministry.
"There was no ambiguity in the trial procedures - and there was no pressure to get confessions from the people on trial," he said on Monday.
Reports speak of a febrile atmosphere in Iranian political circles, as the leadership continues to face overt criticism over its handling of the post-election crisis.
One focus of controversy has been the conditions under which remaining the detainees have been held, with damaging claims forcing authorities to act.
The Supreme Leader, Ayatollah Ali Khamanei, closed the notorious Kahrizak detention centre over allegations of abuse, and both the Iranian parliament and judiciary have established committees to investigate the post-election unrest and the government's response.
Official election results awarded incumbent President Mahmoud Ahmadinejad a sweeping victory in the polls.
He is in the process of selecting a cabinet which will be submitted to parliamentary approval next week.