Myanmar has started releasing prisoners under an amnesty in which more than 450 will be freed.
The government announced the move days before a visit by US President Barack Obama.
The releases are the latest in a series under the government of President Thein Sein.
Since the end of military rule in 2011, hundreds of dissidents have been freed.
The last amnesty in September saw the release of more than 500 prisoners. But it is estimated that at least 300 are still in prison.
The BBC reports many political detainees are in remote areas, so the full extent of the releases may not be known for some time.