Nearly 300 friends and sympathizers attended the funeral of a 34-year-old pro-democracy activist who had been imprisoned by the military junta and died this week in jail, a colleague said Wednesday.

Thet Win Aung, who had been serving a 59-year sentence since 1998 after protesting for educational reform, died Monday in a jail in central Myanmar, formerly known as Burma.

Amnesty International called on Myanmar to launch an independent investigation into his death, saying he was tortured during his imprisonment and suffered from health problems, including malaria. By 2005, he was reported to have been unable to walk without assistance.

Myanmar's military government said he died of natural causes.

"Torture is not our policy and we have strict rules and regulations which forbid abuse of prisoners," Ye Htut, an Information Ministry official, said Tuesday in an e-mail.

Mya Aye, a former student leader who attended the funeral, said Thet Win Aung's family was informed by prison authorities that he died of "heart failure."

CountryWatch: Myanmar

The funeral, held at Kyarnikan cemetery in the central city of Mandalay, was attended by Thet Win Aung's parents and elder brothers, as well as scores of friends and members of the National League for Democracy, Mya Aye said.

The National League for Democracy, led by Nobel peace laureate Aung San Suu Kyi, is the main political party opposing the military government.

Another of Thet Win Aung's brothers, Htay Win Aung — a key leader of the dissident group 88 Generation Students — was arrested on Sept. 30 along with four other student activists. The brother, better known as Pyone Cho, had sent a letter to the ruling junta chairman inquiring about the status of three other detained activists.

Pyone Cho, who is still detained, was among the leaders of a 1988 nationwide uprising, which the military crushed before seizing power. He served nearly 14 years in prison and was released in 2003.

The junta held a general election in 1990 but refused to recognize the results after a landslide victory by Suu Kyi's National League for Democracy.

Myanmar's military government is said by human rights groups and the United Nations to hold more than 1,100 political prisoners under inadequate and brutal conditions. The Assistance Association of Political Prisoners, a group of former political prisoners based near the Thai-Myanmar border, said more than 125 political prisoners have died in Myanmar's jails since 1988.

Australia expressed sadness Wednesday over Thet Win Aung's death, calling on Myanmar's government to carry out an independent investigation and release all political prisoners.