Iran is reported to have freed at least 11 political prisoners, including noted human rights lawyer Nasrin Sotoudeh.
The eight women and three men are said to also include the reformist politician Mohsen Aminzadeh.
The move comes just days before Iran's new President Hassan Rouhani visits New York for the UN General Assembly.
In another development, Mr Rouhani described a recent letter sent to him by US President Barack Obama as "positive and constructive".
In an interview with US broadcaster NBC, he said he had full authority to negotiate a deal over Tehran's controversial uranium enrichment programme with the West.
