Burma's ruling military junta has freed its second-most-prominent political prisoner, Min Ko Naing, a key leader of 1988 student democracy protests that led to a bloody crackdown and mass arrests.

His release Thursday after 15 years in prison could signal that Burma's generals are willing to pursue political reforms, some specialists on the country said. But activists were skeptical, saying their hopes for change often have been frustrated.

Aung San Suu Kyi, Burma's most prominent political prisoner, who won the 1991 Nobel Peace Prize while in detention, remains under house arrest. The United Nations, United States and European Union consider her release essential toward reform.

Naing, 42, was freed from a prison in Sittwe in northern Burma and flown 350 miles southeast to Rangoon, the capital. He had a tearful reunion with his family, said Aung Din, a fellow former student leader who now directs policy for U.S. Campaign for Burma, a Washington-based activist group.

Naing was among at least 19 political prisoners released this week, including at least three senior members of Suu Kyi's National League for Democracy party, according to activists in the United States and Thailand.

State-run media reported late Thursday that the junta had begun releasing 3,937 prisoners who were wrongly charged by the former National Intelligence Bureau. Most are not political prisoners, the activists said.

Aung Din said the release could be designed to ease international pressure, particularly since the sacking last month of Gen. Khin Nyunt, Burma's prime minister and number three official in the military hierarchy. He had been considered willing to work with the international community on a gradual move toward democracy.

But activists have charged that such moves are meaningless without Suu Kyi's release. They said that so far, fewer than 1 percent of Burma's 1,500 political prisoners have been released.