Despite the pardons, critics note that people continue to be locked up for political offenses under the nominally civilian government that took office in early 2011.
"Today's release is of course welcome, but the fact remains that there are many imprisoned for peaceful activism still behind bars in Myanmar," said Isabelle Arradon, Amnesty International's Asia-Pacific deputy director.
"We continue to receive reports of peaceful activists and human rights defenders being harassed and at risk of imprisonment for nothing but expressing their opinions," she said. "This has to end immediately, otherwise, releases like the one today will be meaningless."
Ye Aung, himself a former prisoner, said after Friday's announcement that at least 60 political prisoners remain in jail.
Dozens of those released Friday had been charged under Section 18 of the Peaceful Assembly Law for staging protests without getting prior permission, he said.
Well-known activist Naw Ohn Hla, who was handed a two-year sentence for causing public panic, was another of the more prominent inmates released.
She was detained in the Sagaing region in August for asking to protest a controversial copper mine in the area and the abrogation of the 2008 constitution.
During his visit to France in July, Thein Sein pledged that his government will free all political prisoners by the end of the year.