Student democracy leader Min Ko Naing said he was set free in western Myanmar on Friday night and flown back to the capital Yangon to reunite with his family after spending 16 years in detention.
The move was part of a mass release of 3937 prisoners who the government admitted may have been wrongly detained, and which included several senior National League for Democracy members and other foes of Yangon's military rule, NLD spokesman U Lwin said.
No official word has been given of exactly who was released.
But NLD sources said at least two dozen NLD members and other pro-democracy figures were now believed to have been freed in the move which comes less than two weeks ahead of a South-East Asia summit where criticism of Yangon's military rule is expected.
The NLD, whose charismatic leader Aung San Suu Kyi remains under house arrest, said in a statement that the government's release of political dissidents lent dignity to the nation. It praised those freed for their continued struggle for democracy.
Min Ko Naing, appearing frail but excited and surrounded by relatives and stunned friends, said he needed time during his new-found freedom to deliberate about how, if at all, he would engage in the pro-democracy movement.
Myanmar has been blacklisted for
human rights abuses
"I will take my time to consider matters before making any decisions," he said from his home.
Second only in stature among detained dissidents to Aung San Suu Kyi, Min Ko Naing - whose real name is Paw U Tun - became one of the most prominent figures to emerge from the pro-democracy student protests he led which were brutally crushed by Myanmar troops in 1988.
He was arrested the following year for anti-government activities and had remained behind bars ever since, rights watchdog Amnesty International said.
In a 1988 speech, the fiery student leader said: "If we want to enjoy the same rights as people in other countries, we have to be disciplined, united and brave enough to stand up to the dictators," according to Amnesty.
The military has ruled Myanmar, formerly known as Burma, since 1962 despite the National League for Democracy winning a landslide election in 1990.
The NLD's election victory in 1990
was not allowed to bear fruit
Min Ko Naing's release was interpreted by NLD spokesman U Lwin as an important gesture by the government as it could serve as a barometer of its willingness to engage in political reform.
He said it also boosted chances for reigniting United Nations-brokered national reconciliation talks which were secretly launched between Aung San Suu Kyi and the government's General Khin Nyunt but which had collapsed last year.
"I think this is an expectation and we are looking forward to starting off negotiations for reconciliation," U Lwin said, although he stressed the government had made no direct overtures about resuming talks.
Several NLD sources said senior party member and pro-democracy journalist Win Tin was among those freed.
The award-winning writer, aged 74 and in poor health, has spent the past 15 years in detention, with several international human rights groups and the UN demanding his release.
But the detainee's nephew confirmed he was not freed, as he visited Win Tin at Insein prison on Saturday.
The UN said it was encouraged by the releases but wanted to see the government free all political prisoners.