Dozens of women were among 240 detainees who were released yesterday, said attorney and leading human rights activist Michel Shammas.
Most of the prisoners in Syrian jails being prosecuted under "anti-terror" laws are opponents of the Syrian government or those who have taken part in the uprising that began in March 2011.
The Syrian Observatory for Human Rights, a Britain-based monitor, also reported the releases, but put the number of freed detainees at around 350.
"Syrian authorities released more than 240 prisoners from Adra prison (near Damascus), most of whom were tried before what is know as the anti-terrorism court," Shammas told AFP late yesterday.
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Among those freed was prominent Syrian blogger Hussein Ghreir, who was arrested in February 2012 along with fellow activists Mazen Darwish and Hani Zaitani and accused of "promoting terrorist acts."
A friend of Ghreir's, who spoke on condition of anonymity, confirmed that the blogger was "at his home in Damascus and is in good health".
Among them are thousands of people who have effectively disappeared since being detained, with their families unsure where they are being held.
According to the Observatory, nearly 13,000 Syrians, including dozens of children, have been tortured to death in government prisons since March 2011.
Last year, President Bashar al-Assad signed an amnesty that was supposed to see tens of thousands of political prisoners freed, but rights activists say that only several hundred were actually released.