Dozens of Iranian security forces have stormed a section of Tehran's Evin prison where political prisoners are held, injuring a number of the detainees.
Some 30 prisoners were transferred to solitary confinement following the raid on April 17.
The reports by opposition websites have been confirmed by some of the families of the prisoners.
The daughter of jailed human rights lawyer Abdolfatan Soltani told RFE/RL's Radio Farda that her father was among the prisoners who were beaten up and isolated.
"About 100 guards, members of antiriot forces, entered the prison. Under the excuse of inspecting Section 350, they brutally attacked the prisoners and beat them up," Maedeh Soltani said in telephone interview from Tehran.
She said she received the information from "reliable" sources inside the prison.
The website Kalame, which broke the news, says the "unprecedented" raid is the most violent action against political prisoners in the past 20 years.
"The raid took place after prisoners protested and resisted against a brutal inspection plan. It lasted for five hours. The guards took pictures and filmed their operation," the website said in its report.
The report said more than 30 prisoners were injured, including four who were transferred outside of the prison to receive medical care. According to the report, some of the prisoners sustained injuries, such as a cracked skull and broken ribs.
The opposition Sahamnews reports that at least two prisoners -- national religious activist Emad Bahvar and blogger Hossein Ronaghi Maleki -- were seriously wounded in the attack.
Gholamhossein Esmaili, the head of Iran's Prisons Organization, dismissed the reports as "rumors."
"We shouldn't pay attention to report by antirevolutionary media," Esmaili was quoted as saying by the semiofficial ILNA news agency.
Amnesty International says reports about the raid have raised fresh fears about the safety of prisoners in Iran.