The amnesty declared by President Bashar al-Assad came a week after his controversial re-election as he seeks to portray himself as the champion of reconciliation in the war-torn country.
Assad is due to be sworn in for a new term on July 17.
"This is the most important amnesty since Hafez al-Assad (the president's father and predecessor) came to power nearly 45 years ago," said human rights lawyer and ex-prisoner of conscience Anwar al-Bunni.
According to Bunni, "dozens of prisoners began to be released from Adra prison (in Damascus province) yesterday and the releases will continue today."
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State television showed dozens of prisoners being freed in Hama in central Syria.
The amnesty is unprecedented because it extends for the first time to those accused under the country's anti-terrorism legislation.
The government has dubbed all of those opposed to Assad's rule -- armed opposition fighters and peaceful activists alike -- of "terrorism", and used the law to imprison high-profile dissidents.
Army deserters will be given full pardons if they hand themselves in within three months of the decree, according to the text.
But it was unclear how many prisoners might be freed under the amnesty, as previous clemency decisions have not seen large numbers of detainees released.