Protests have continued since the 12 June presidential election
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.
But the BBC's Jon Leyne says the opposition will be sceptical of the change of heart - though there are many relatives of prisoners desperately anxious to be reunited with their loved ones.
Officials also announced on Tuesday that 30 people were killed in clashes between opposition supporters and police - up from a previously stated figure of about 20.
Speaking of the prison releases, National Security and Foreign Policy Committee member Kazem Jalali said those released had committed "lighter offences".
Jon Leyne, Tehran correspondent
The issue of opposition detainees arrested since the election has become the new focus of criticism of the Iranian government, even from former government supporters.
Accounts from the opposition suggest that, at best, the prisons are unable to cope; at worst, there is systematic abuse of those detained.
The government has always seen compromise as a sign of weakness, but it has finally been forced to give ground. The opposition will suspect that the closure of a prison and release of detainees is purely cosmetic, not a real change in policy.
For the supreme leader it is a new humiliation. He has been reduced to ordering prison closures himself - usually the work of the president or the interior minister. With the president's inauguration for his second term expected in a few days, the pressure on the government can only increase.
No well-known political figure was among the people released on Tuesday, he said.
The parliamentary committee is investigating the detentions.
The prisoner releases came after the committee visited the prison, Mr Jalali said.
Mr Jalali, quoted by Fars news agency, 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 described by judicial officials as the "agents of the unrest and some of them were members of anti-revolutionary groups".
These 200 cases were still being investigated, Mr Jalali said.
But one report suggests President Mahmoud Ahmadinejad has now ordered the swift release of all political detainees not facing serious charges.
"We want their families to be happy by their release," AFP news agency quoted him as saying in a letter to Ayatollah Mahmoud Hashemi Shahrudi, head of the country's judiciary.
"They should be home for the occasion of the birth anniversary of Imam Mahdi which falls on 7 August."
Supporters of the protesters say the true number of people detained may be in the thousands.
Memorial rally rejected
They have also cast doubt on the number of people killed, believing it to be closer to 100.
Opposition candidates Mir Hossein Mousavi and Mehdi Karroubi requested permission to hold a memorial rally for the dead on Thursday but were refused by the interior ministry, Fars news agency reports.
Another detention centre, Kahrizak, was ordered to be closed "because it lacked necessary conditions to preserve rights of detainees", Mr Jalali was quoted as saying earlier by the semi-official Mehr news agency.
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.
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