The Burmese government has freed about 200 political prisoners as part of a general amnesty, activists say.
A popular comedian and dissident, Zarganar, was among the first to be freed. Some monks and a journalist were also released.
Burmese pro-democracy leader Aung San Suu Kyi welcomed the amnesty.
The country's continued detention of about 2,000 political prisoners is a crucial reason why Western nations maintain sanctions on Burma.
The detainees include journalists, pro-democracy activists, government critics, monks involved in anti-government protests and members of Burma's ethnic groups fighting for greater autonomy.
Zarganar was arrested in 2008 after publicly criticising the government's response to Cyclone Nargis, which killed more than 140,000 people.
He organised aid runs and collections amid international anger at the junta's lack of action. He was convicted of illegally giving information to the press and causing public alarm.
Speaking to the BBC shortly after his release, Zarganar was wary of his new-found freedom, describing it as conditional.
"If I do something wrong they will send me back. I'm not happy today because there are so many of my friends still in prison," he said.
Several hundred political prisoners remain behind bars, including some of the most high profile activists.
Leaders of a failed uprising in 1988 are reportedly still in jail.
A Burmese prison official told the BBC that 300 dissidents had been freed, but this figure has not been confirmed.
Many of those who were released from Insein prison, where many of the political detainees are held, were elderly or infirm.
An ethnic minority militia commander, Sai Say Htan, of the Shan State Army, was among those freed.
Rights group Amnesty International said Burma's government - a nominally civilian administration dominated by leaders of the former military regime - must free more political detainees if it is seriously committed to reform.
"This release of political prisoners is welcome, but is not consistent with the authorities' recent promises of political reform in Burma," said Benjamin Zawacki, Amnesty International's Burma researcher, who is based in Bangkok.
"Unless the figure rises substantially, it will constitute a relaxation of reform efforts rather than a bold step forward."
Britain described the releases as an encouraging first step, and the European Union said Burma was "on the cusp of change".
The government said on Tuesday that more than 6,000 prisoners would be freed but it was unclear how many would be political detainees.
Ms Suu Kyi, herself freed from 15 years of house arrest last year, said: "We hope many more will be released. I'm really thankful for the release of political prisoners."
Early reports that one of the monks' leaders, Shin Gambira, had been released, have now been denied.
He led street protests in 2007 in what became known as the Saffron Revolution, an uprising that was crushed by the previous military government.
Barometer of change
Burma announced an amnesty of 15,000 prisoners in May 2011 and freed more than 7,000 in 2009 - but those moves were criticised by rights groups for failing to include political prisoners.
Burma held its first elections in two decades almost a year ago - polls which saw military rule replaced with a military-backed civilian-led government.
Since then the government has freed Aung San Suu Kyi and held a dialogue with her.
But Nyan Win of Aung San Suu Kyi's National League for Democracy (NLD) told the BBC that a prisoner release was not all that was needed.
"The release of political prisoners is just one of the barometers of the government's seriousness about a change to democracy," he said.
"There should be other developments like media freedom, and the relaxation of censorship among other things."