Speaking to Aljazeera.net on Thursday, the secretary general of Solidarite Tunisienne, Fathi Ennaes, alleged that Nabil al-Waair has suffered sexual and physical abuse in Burj al-Rumi jail over a sustained length of time.
The France-based group has also named several of the officers involved and demanded that a thorough investigation takes place.
"I believe it is only right that the mistreatment dealt out to political prisoners held in dire circumstances should be made as public as that at the Abu Ghraib prison in Iraq," Ennaes said.
"The Tunisian government should not take Abu Ghraib as a green light to torture its own prisoners."
One of nearly 600 political prisoners held in the north African country, al-Waair was given a 15-year sentence by a military tribunal in 1992 for belonging to the Nahda political party.
A popular Islamist party, it was accused of plotting to overthrow President Zain al-Abidin Bin Ali - who had himself taken control of Tunisia after forcing president-for-life Habib Burghiba from power in 1987.
Al-Waair's mother, Halima al-Jalasa, told Aljazeera.net that she found out about her son's torture after being denied permission to visit him for three weeks in June.
"Nabil told me he had been tied to the ceiling and left for hours, he has had his feet whipped, he has been beaten. He needs help. It is just intolerable that anyone can be treated like this."
When contacted by Aljazeera.net, no spokesman for the Burj al-Rumi prison was prepared to comment on the allegations.
An international human rights group has also raised several concerns about the situation in Tunis in a 2004 report.
Based in London, Amnesty International said that torture continued to be reported, even cases of mistreatment on the premises of the Ministry of the Interior.
Tunisian rights lawyer Radhia
Nasraoui: Tunisia is a police state
The report lists the names of some of the hundreds of political prisoners, including prisoners of conscience, many of whom have been held for more than a decade.
The rights watchdog also alleges that suspected political opponents of the government continue to face unfair trials, often resulting in long prison sentences.
Even released political prisoners continue to be subjected to administrative control and other arbitrary measures, affecting their freedom of movement and right to work.