The amnesty came as democracy champion Mr Suu Kyi arrived Monday for a keenly anticipated US tour which will coincide with a separate visit next week to New York by President Thein Sein.
"About 60 political prisoners have been released under an amnesty. According to our figure, we have about 300 political prisoners inside the prisons," said the NLD's Nine Nine, adding the party was trying to confirm releases around the country.
The figure was broadly similar to that from Thailand-based Assistance Association for Political Prisoners, which collates information on jailed dissidents in Myanmar.
A prisons department official in the capital Naypyidaw told AFP that a total of 514 detainees had already been freed, the majority of whom were foreigners.
"Altogether 399 foreign prisoners were released under the amnesty yesterday. They were Chinese, Indian, Bangladeshi, Thai and Laotian," he said, declining to comment on whether the remainders were political prisoners.
Thailand's foreign ministry said 83 of its nationals who were arrested in July after illegally crossing into Myanmar were also among those to be freed.
Explaining the pardons, state television Monday said they were aimed at bolstering the "stability of the state and eternal peace, by respecting humanitarian grounds... and also to have friendship and goodwill in relations with neighbouring countries".
Estimates of the exact number of political detainees still locked up vary but opposition groups have said around 300 activists still languished in jails around the country until this week.
Rights groups welcomed Monday's amnesty but called for further details over exactly who was being released and how many political prisoners remain in jail.
Myanmar was for decades ruled by an iron-fisted junta, but a reformist government under ex-general President Thein Sein has ushered in sweeping changes, including welcoming Mr Suu Kyi's party back into mainstream politics.
But activists say steps towards greater democracy are undercut by the remaining detentions of dissidents.
Mr Suu Kyi, who is now an elected politician, used her long-awaited acceptance speech for the Nobel Peace Prize in Norway in June to call for the release of Myanmar's remaining political prisoners, warning of the risk that "the unknown ones will be forgotten".
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