Human rights lawyer Nasrin Sotoudeh, seen by campaign groups as Iran's highest profile political prisoner, has been released from prison.
Other prisoners linked to the mass protests after the disputed 2009 re-election of President Mahmoud Ahmadinejad were also freed, opposition website Kaleme reported.
"I am free from prison today and I am glad but I am worried for my friends in prison," Ms Sotoudeh told CNN from Tehran
She said was "free forever", not on temporary release, and planned to resume her legal career.
Arrested in September 2010, Ms Sotoudeh was serving a six-year term for spreading propaganda and conspiring to harm state security.
Ms Sotoudeh, 50, who defended journalists and rights activists, including Nobel Peace laureate Shirin Ebadi, went on hunger strike for nearly 50 days last year to force authorities to repeal a travel ban on her young daughter.
With fears that she might die, the United States was among the countries criticising Iran and demanding Ms Sotoudeh be freed.
The prison releases come less than a week before President Hassan Rouhani addresses the UN General Assembly for the first time and is expected to present a less confrontational image than Mr Ahmadinejad, under whose eight years in power Iran came under ever-tougher Western trade sanctions.
Mr Rouhani won a surprise victory over hardline rivals in June, pledging to ease some political and social restrictions, and his supporters have called for the release of political prisoners.
Kaleme said one of those released was Mohsen Aminzadeh, a former deputy foreign minister under reformist former president Mohammad Khatami, who supported reformist presidential candidate Mirhossein Mousavi in 2009.
Feizollah Arabsorkhi, a former deputy minister of commerce under Mr Khatami, was also released, the website said.
The total number of prison releases was not immediately clear, but various news reports mentioned seven other women and three men in total.
Ms Sotoudeh's release comes one day after Supreme Leader Ayatollah Ali Khamenei said he supported "flexibility" in Iranian diplomacy, a rare signal that he might endorse a shift in Tehran’s stance in its problematic relations with the West.
Authorities, who transferred Ms Sotoudeh from Tehran's Evin jail to her home, gave no reason for the release and offered no details on who had ordered it, Ms Sotoudeh's husband Reza Khandan said.
"They just told her: 'You're free, go'," he said.
When asked if her release signalled a "new day" for Iran, Ms Sotoudeh told CNN: "It is soon to say new day because we have many political prisoners in prison, but I hope this will be a new day."