President Thein Sein signed a clemency order on Monday marking this week's 64th anniversary of independence. He said the sentence reductions were "for the sake of state peace and stability" and on "humanitarian grounds."
Under the order, death sentences will be commuted to life imprisonment, and prisoners serving more than 30 years will have their sentences cut to 30 years. Those serving 20 to 30 years will have their terms reduced to 20 years, while those with less than 20 years will have their sentences cut by one-fourth.
Most political prisoners, both from the pro-democracy movement and from out-of-favor government factions, are serving long terms and will remain in prison.
The United States said Tuesday it understood that only about 10 prisoners had been released because of the clemency order. State Department spokeswoman Victoria Nuland said the U.S. remained concerned about the more than 1,000 political prisoners still held.
She said that Secretary of State Hillary Rodham Clinton had made clear to Myanmar's government during a landmark visit to the country in December that it would be difficult for the U.S. to make significant steps toward normalization of relations without prisoner releases.
Nuland told reporters in Washington the clemency order was "not a step of the magnitude that we would be interested in matching."
Last week, Myanmar state-run media reported that the government-appointed National Human Rights Commission had appealed to Thein Sein to issue a general amnesty for prisoners.
Thein Sein has pushed forward reforms since taking office last March, following decades of repression under previous military regimes. His government is still dominated by a military proxy party, but changes have been made in areas such as media, the Internet and political participation.
"I am very disappointed and feel hopeless because the clemency order makes no difference to political prisoners. Only common criminals will be freed," student activist Phyo Min Thein said Tuesday.
He said the decision will cause a loss of confidence in the 10-month-old nominally civilian government.
Phyo Min Thein was released from prison in 2005 after serving 15 years. His brother-in-law, activist Htay Kywe, had his 65-year sentence reduced to 30 years with Monday's order, but still has 26 years to serve.
"What families of political prisoners want is absolute freedom," said Kyi Kyi Nyunt, the sister of prominent student activist and political prisoner Min Ko Naing, whose 65-year term was also reduced to 30 years.
"It is very disappointing and devastating. It is clear that political prisoners will not be freed for a long time," said Win Tin, 82, a prominent journalist and member of pro-democracy leader Aung San Suu Kyi's National League for Democracy party.
"Such a move can cause political instability and the tempo of political demands will increase," he said.
The prisoners reportedly freed Tuesday, including 11 political detainees, had little time remaining on their sentences.
Well-known comedian and social critic Zarganar Thura said the government failed to deliver on its promise.
"I once likened the situation of my friends in jail as being in the hands of Somali pirates. I now withdraw this comment. The Somali pirates keep their promise," he said on his Facebook page.
Zarganar was released from three years in prison in an amnesty last October that freed 6,359 prisoners, including about 200 political detainees.
Suu Kyi said in November that there were still about 600 political prisoners, but some human rights groups put the number at about 1,500.
The release of political prisoners has been seen as a crucial step in the country's reforms, and a failure to free a substantial number is also likely to be considered inadequate by the European Union and other nations.