Myanmar on Tuesday announced there were "no more political prisoners" after issuing a sweeping amnesty order aimed at fulfilling a presidential pledge to free all dissidents by the end of the year.

The country has released scores of prisoners of conscience as part of dramatic reforms, implemented since the end of outright military rule in 2011, that have ended the former pariah's international isolation and seen most western sanctions disbanded.

It was not immediately clear whether the amnesty would affect all of the approximately 40 political prisoners listed by campaigners, as well as a further 200 people awaiting trial, mainly for protesting without permission.

Myanmar late Monday said it would pardon those imprisoned under controversial legislation, including the Emergency Act used by the junta to imprison opponents as well as laws governing freedom of assembly and the right to protest.

Presidential spokesman Ye Htut said the amnesty, along with a separate pardon for five additional inmates jailed under other legislation, meant "there are no more political prisoners".

"I would like to say that the president has fulfilled his promise given to the people, because there will be no political prisoners at all at the end of 2013," he said in a post on his Facebook page, without giving further details of the release, which began Tuesday.

Former general Thein Sein, who has won international plaudits for overseeing new political and civil freedoms since becoming president nearly three years ago, had vowed to free all detained activists by the end of the year.

But campaigners sounded a note of caution over the announcement.

David Mathieson, a researcher with New York-based Human Rights Watch, said the claims could be "bluster" to try to meet the self-imposed release deadline.

"They can claim a technical fulfilment of the presidential pledge at best," he told AFP, adding the government would need to demonstrate it had resolved the cases of the dozens of people facing trial.

He also called on Myanmar to repeal the controversial laws, with campaigners fearing authorities could continue to arrest critics in the future, potentially creating more political prisoners.

Dozens of relatives and friends gathered outside Yangon's notorious Insein prison early Tuesday awaiting news of their loved ones.

Peace activists Yan Naing Tun and Aung Min Oo, who were sentenced in recent weeks to eight months in prison for marching to the rebel town of Laiza in strife-torn northern Kachin state, were greeted by jubilant supporters as they walked free from the jail.

"I respect the president for keeping his promise," Yan Naing Tun told reporters.

Arbitrary imprisonment was a hallmark of nearly half a century of harsh rule by a junta that denied the existence of political prisoners, even as it imposed harsh punishments on rights activists, journalists, lawyers and performers.

Before Myanmar's reforms, rights groups accused the country of wrongfully imprisoning about 2,000 political detainees -- most of whom have since been freed.

Prison authorities said they did not have a timetable for releasing those pardoned under the latest amnesty, adding it could take time to identify all those affected.

"We do not know how many yet as we are still scrutinising the list... we will release them as soon as possible," said corrections department director Than Htay.

The latest amnesty includes the most notorious laws used by the former junta against its critics, including democracy campaigner Aung San Suu Kyi -- who is now an opposition MP following the country's dramatic reforms.

While it appears to cover those currently facing charges, it does not extend to people arrested after December 31.