"As far as I know, about 120 political prisoners have been released. We are still compiling the list," said Nyan Win, spokesperson of long-term dissident Aung San Suu Kyi's National League for Democracy.
State television announced on Tuesday that more than 6,300 elderly, sick, disabled or well-behaved prisoners would be granted an amnesty from Wednesday "on humanitarian grounds".
It said freeing detainees would allow them to "help to build a new nation".
President Thein Sein, a former general and senior junta figure, has surprised critics by signalling a series of political reforms since taking power following a controversial election last November.
Last month the government scrapped a dam project, saying the the move was because of public opposition.