About 140 Iranians detained during protests against last month's disputed election result have been released from Evin prison, officials say.
About 200 others, accused of more serious crimes, remain in the prison.
The release comes after Iran's supreme leader ordered the closure of another detention centre because it failed to "preserve the rights of detainees".
The unusual moves show how much pressure Iran's leaders are under over detainees, correspondents say.
A spokesman for the National Security and Foreign Policy Committee, Kazem Jalali, quoted by Iranian news agencies, said: "Those who were released had committed lighter offences."
No well-known political figure was among the people released on Tuesday, he said.
The committee was set up to investigate the detentions, with the leader of the judiciary ordering a review of all cases of those held in prison since the election.
'Agents of unrest'
Mr Jalali, quoted by Fars, said 150 people who remain in prison were suspected of carrying weapons and bombs, and vandalising public property during the unrest.
A further 50 prisoners were, according to judicial officials, the "agents of the unrest and some of them were members of anti-revolutionary groups," he was quoted by Fars as saying.
The 200 cases were still being investigated.
Supporters of the protesters say the true number of people detained is much higher.
The releases from Evin came after a visit to the notorious prison by the committee, he said.
Meanwhile, another detention centre - Kahrizak - was ordered closed "because it lacked necessary conditions to preserve rights of detainees," Mr Jalali was quoted earlier by the semi-official Mehr news agency as saying.
It is not clear whether the detainees at the Kahrizak centre were released or transferred elsewhere.
In recent days the opposition has reported almost every day new deaths of protestors held in prison.
Iran's prisons are notorious for their poor conditions, correspondents report.
Former political prisoners, such as journalists and bloggers, have complained of human rights abuses such as solitary confinement, harsh interrogation tactics and even torture at Evin.