The Human Rights Association of Syria said on Monday: "On the occasion of the day of the Syrian political prisoner, the association asks the relevant authorities to free immediately political prisoners and prisoners of conscience, shed light on the fate of those people who have disappeared, and ban all forms of physical and moral torture."
It said "many detainees have been incarcerated for 25 years, the fate of thousands of detainees is unknown, dozens suffer incurable sickness, some have been tried by iniquitous special tribunals, others have been tortured."
Estimates for the number of political prisoners vary, with some sources suggesting 800 and others more than 2000.
The association's call coincided with a plea by another group, the Committees for the Defence of Human Rights and Democratic Freedoms in Syria, for the authorities to release its detained leader Aktham Naysse who had gone on hunger strike.
It said Naysse started his strike on Sunday to protest against his "detention and the conditions of his detention, which constitute a serious health hazard for him."
The organisation asked that Naysse, who is also a lawyer, be released immediately. It was informed of the hunger strike by Naysse's family, who visited him briefly on Sunday.
Naysse suffers from heart and kidney problems.
He was arrested in Latakia, northwest Syria, and is due to appear in a special state security court accused of carrying out activities which run counter to the country's socialist system.