Among the freed detainees are former leader General Ne Win's grandchildren, who were sentenced to death 11 years ago for an alleged coup attempt, the BBC reported.
Also freed was Naw Ohn Hla, a female environmental activist sentenced to two years in jail in August for opposing a controversial Chinese-backed copper mine.
The BBC said the releases signal progress on the promise by President and former junta leader Thein Sein to release the detainees. More than 1,000 political and environmental activists were in prison three year ago.
Human rights group Amnesty International welcomed the latest release of prisoners, but warned time is running out for the government to make good on its promise.
The predominantly Buddhist country of 60 million has been opening up to foreign investment and tourism since the generals' relinquished power after half a century of dictatorship.
The junta was replaced by a military-backed civilian government of former officers that won an election in late 2010.
Since taking office in early 2011, the government also has loosened media restrictions and released hundreds of political detainees.
But the government has been struggling with allegations of human rights abuses, notably against minority groups and rebels.
"Today's release is of course welcome, but the fact remains that there are many imprisoned for peaceful activism still behind bars in Myanmar," Isabelle Arradon, Amnesty International's Asia-Pacific Deputy Director, said.
"President Thein Sein has promised to release all prisoners of conscience by the end of the year, but time is running out to show that this was not just empty words.
"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. This has to end immediately, otherwise releases like the one today will be meaningless," Arradon said.
Among the released prisoners is Naw Ohn Hla, an ethnic Karen woman who was imprisoned in August for two years of hard labor for peacefully protesting a copper mining project.
"Naw Ohn Hla should never have been locked up in the first place and the fact that she has been given a conditional release and is still facing other charges is not good enough. She and others like her should be released unconditionally," Arradon said.
A particular flashpoint in recent years has been ethnic relations in the northwestern state of Rakhine where Rohingya -- a Muslim group with close ties to neighboring Bangladesh -- have clashed with Buddhists.
Communal violence in Rakhine in May last year left nearly 80 people from both communities dead. More than 100 people were injured and nearly 5,000 homes, 17 mosques, 15 monasteries and three schools were burned, the government said at the time.
Amnesty said among those remaining in prison are Tun Aung, a Rohingya Muslim serving 17 years for peacefully trying to halt communal violence last year, and Kyaw Hla Aung, a human rights activist detained since July who is currently on trial and facing a lengthy prison sentence.
Arradon called for their immediate release, saying they are behind bars for exercising their right to freedom of expression.
"Myanmar must ensure that throughout its transition and beyond there is space for civil society and for human rights defenders to work free from harassment and threat of criminalization."
Bart Vermeiren, deputy director of the International Committee of the Red Cross in Myanmar, said lingering religious tensions in Rakhine state are contributing to a humanitarian crisis.
"Livelihoods have been severely damaged and access to clean water and healthcare has been significantly reduced," he said in a statement Thursday.
Piebalgs, as part of the first European Union and Myanmar task force to encourage national development, said EU support for education, peace building and rural development in Myanmar could reach $120 million per year.
"The [political] development taking place in Myanmar is unprecedented and needs to be acknowledged," he said.
"But we must not forget about the challenges ahead, for which the European Union, as one of the main donors, will stand by with further support to continue the necessary reforms in the country."
Clinton, as head of the Clinton Foundation, met with government officials, including Thein, to discuss Myanmar's peace-making process, agricultural development, healthcare issues and power generation.