The Thailand-based Assistance Association for Political Prisoners also said that security officers had been threatening dissidents' relatives and neighbors to find information on the whereabouts of those involved in recent mass protests against 45 years of military rule.
"The security forces have become more severe in raiding houses of, and searching for, anyone whom they suspect to have been involved in the protests," it said.
The Myanmar exile group, made up of former political prisoners, said authorities had recently informed the family of NLD member Win Shwe, 42, that he had died under interrogation in the central Myanmar region of Sagaing. He and five colleagues were arrested on Sept. 26.
His body was cremated at the detention center, the group said. The report could not be independently verified, although in the past the group has provided detailed, accurate information on political prisoners in the country.
The group said that at least five people have been arrested over the past two days in Yangon, even as the junta and pro-democracy leader Aung San Suu Kyi's National League for Democracy appeared to be taking cautious steps toward talks.
In the face of international pressure, the junta's top general offered last week to meet with Suu Kyi, a Nobel Peace laureate who has been under house arrest for 12 of the past 18 years without trial. He said she must first renounce her calls for international sanctions against the regime, which has been widely condemned for crushing last month's protests.
The junta also appointed a relatively flexible Cabinet-level official to coordinate contacts with Suu Kyi, 62, whose league said it was prepared to make "adjustments" for the sake of dialogue.
The appointment of Aung Kyi, a retired major general, appeared to be a gesture toward the United Nations. The world body's special envoy, Ibrahim Gambari, suggested creating the Cabinet-level job during his visit to Myanmar last week, state media said.
Small groups of riot police Wednesday patrolled key road junctions and sites where the most intense protests had erupted, including the Shwedagon Pagoda and a junction near the downtown Sule Pagoda. But soldiers were not visible on the streets, and Yangon seemed generally normal.
The state-owned New Light of Myanmar newspaper said 60,000 pro-government demonstrators had gathered Tuesday in Paan in eastern Myanmar to support the junta's own "roadmap to democracy" while denouncing the United States and foreign radio stations.
The Southeast Asian country has been ruled by the military since 1962, and protests that broke out in August over a fuel price hike quickly ballooned into mass demonstrations calling for democracy when widely respected Buddhist monks began spearheading daily marches.
Troops crushed the protests by shooting at demonstrators Sept. 26-27. The regime said 10 people were killed, but dissident groups put the toll at up to 200 and say thousands of students, Buddhist monks and others were arrested.
The brutal crackdown ignited outrage around the globe.