Win Shwe, a 42-year-old member of the National League for Democracy was arrested with five colleagues on September 26, the day the junta began to put down the demonstrations.
According to a Thailand-based human rights group, the Assistance Association for Political Prisoners, the security forces told Win Shwe's family that he had died during interrogation.
"They didn't say when he died or the reason why he died, but they did say he had been cremated," the AAPP spokesman, Ko Bo Kyi, told Guardian Unlimited.
He described Win Shwe as an "ordinary but active member of the National League for Democracy".
The AAPP is also concerned about the fate of the pro-democracy leader, Hla Myo Naung, who was arrested today while on his way to seek treatment for an eye problem.
Hla Myo Naung is the main spokesman for the 88 Generation Students, a group that takes it name from the 1988 uprising in which 3,000 protesters were killed.
Hla Myo Naung was quoted yesterday by the Irrawaddy, the Thai-based Burmese news website, at the launch of a campaign for the release of political prisoners in Burma.
"It is a peaceful expression and I don't think the authorities will respond to our campaign by punishing participants," Hla Myo Naung said.
The AAPP spokesman said he was concerned that Hla Myo Naung would be tortured.
"His eye problem is serious," he said. "I want the military regime to provide him proper medical treatment and not to torture him. One activist has died under interrogation," he said.
The AAPP claimed that Hla Myo Naung is one of seven people to have been arrested in the last two days.
Meanwhile, a Burmese diplomat resigned in protest against the regime's "horrible" treatment of protesters. Ye Min Tun, a foreign ministry official, announced his resignation in a letter to the Burmese embassy in London. He told the BBC that as a "good Buddhist" he found the beating of monks "horrible".
"This revolution, this incident, seemed to be the decisive factor that could persuade the government to go to the negotiating table. But actually the government ignored the reality."
Burma's military leader, General Than Shwe, has agreed to talk to Aung San Suu Kyi, but only if she agrees to drop her "confrontational stance" towards the regime and stops calling for sanctions against Burma.
Yesterday Aung San Suu Kyi's party rejected his terms for negotiations, but said it was prepared to make "adjustments" for the sake of dialogue.
