The accusations -- in many cases backed up by medical evidence -- brought a wave of official opprobrium on the head of Mehdi Karrubi, the defeated reformist presidential candidate who brought them to light.
Rape after all, is a great taboo subject under an Islamic regime that preaches a tone of piety in sexual, as well as on all other, affairs.
Now fresh allegations have surfaced in an open letter sent by a pro-reformist journalist and Karrubi supporter, Mehdi Mahmudian, to Iran's supreme leader, Ayatollah Ali Khamenei.
The difference this time is that the sexual assault Mahmudian speaks of is not part of some officially sanctioned regime of terror against political prisoners.
Rather, it comes in the form of de facto sex slavery imposed by violent prisoners against inmates unable to defend themselves.
Mahmudian, who has been in custody since September 2009, paints a highly disturbing and terrifying picture of conditions inside Rajai Shahr prison in Karaj, near Tehran.
"Every 'object' has an owner [who] earns money through this and sometimes he is sold to somebody else."
'Raped Seven Times In One Night'
In several sections of the prison, Mahmudian continues, sodomy is a "routine acceptable practice."
Mahmudian is able to describe such scenes apparently because political prisoners are held in the same facilities as common and violent criminals.
Citing the testimony of a fellow political inmate, he describes the plight of a young prisoner who was raped seven times in a single night.
When the victim complained, prison officers hauled him off to a solitary confinement cell instead of taking action against his tormentors.
Addressing Khamenei directly, Mahmudian -- whose letter is dated last September but has only come to public attention in recent days -- writes: "Here in Rajai Shahr prison, which is run under your control, the offspring and citizens of a country under your authority are rented out for 250,000 tomans [about $250] in a system from which the victims do not find any salvation."
Mahmudian, who has reportedly suffered health problems as a result of ill-treatment during his incarceration, also outlines the pervasive presence of drugs and complains about the beating of political prisoners with chains and cables.
Having been an inmate in Tehran's notorious Evin prison and a facility called Kachouyi, as well as Rajai Shahr, Mahmudian says narcotics are freely available in all three in whatever quantity a prisoner wants.
He contrasts this with a six-month wait for clothing items. Many addicted inmates have diseases like AIDS and hepatitis but mix freely with other prisoners, with whom they have sexual relations thus leading to the spread of conditions like HIV.
Mahmudian has blown the whistle on prison conditions in Iran in the past, before he himself was imprisoned.
Latest In A String Of Prison Complaints
He reminds Khamenei in his letter that he drew attention to brutal treatment meted out at Tehran's Kahrizak dentention center in 2007.
The facility was closed on the supreme leader's orders two years later, after several inmates arrested in the postelection protests -- including the son of a prominent government scientist -- died from torture and beatings.
It is the third written complaint about conditions inside Iranian prisons to emerge in recent days.
On May 8, the pro-reformist website, kaleme.com, published an open letter to the head of Iran's Islamic Human Rights Commission, Mohammad Hassan Ziaeefar, from the families of around 600 female political prisoners protesting about Gharchak prison in southern Tehran, where the inmates were denied regular food, water or washing facilities.
And Zia Nabavi, a jailed Iranian student leader wrote last week to Mohammad Javad Larijani, the head of the judiciary's human rights department, about Karun prison in Ahvaz in south-west Iran. He said the jail was overcrowded and dirty and that 1,500 prisoners were forced to sleep outside in a yard regardless of the weather.
-- Robert Tait