Ko Thet Win Aung, 34, was jailed in 1998, sentenced to 59 years for organising student protests over poor quality education and the release of political prisoners.
His father, Win Maung, told the Democratic Voice of Burma - an Oslo-based Burmese media organisation - that his son had died in Mandalay prison last night.
He said while medical records showed his son died from heart failure and a swollen liver, his heart attack may have been caused by hearing the news that his brother, Pyone Cho, also an activist, had been detained again by authorities.
Another brother, Ko Thein Tan, told Reuters Mr Thet Win Aung had been in a "very bad" condition. "He contracted malaria in prison and developed some nervous troubles too and became paraplegic about one year before he died," he added.
Amnesty International in London said today it was "deeply concerned" by the death of the student leader and called on authorities to initiate "a prompt, independent investigation into the causes of Ko Thet Win Aung's death and to make the findings public".
The organisation also urged Burma's military junta, which calls itself the State Peace and Development Council, to take urgent steps to protect all prisoners' health. "Prison deaths, including those of political prisoners, are increasing. Poor prison conditions have further deteriorated during 2006," read the statement.
The Thailand-based Assistance Association for Political Prisoners (AAPP) called the man's death "murderous".
"He wasn't yet 30 when he received a nearly 60-year sentence, and with frequent back and forth transfers from one prison to another, it appears that officials were attempting to murder him," said Tate Naing, secretary of AAPP, to the Irrawaddy, a newspaper written by Burmese exiles in Thailand.
The International Red Cross has been banned since December last year from visiting jails across the country.
Burma has been ruled by the military for over four decades. General elections were held in 1990 and the National League for Democracy, led by Aung San Suu Kyi, won the majority of the vote.
The regime rejected the election results and Mrs Suu Kyi, a winner of the Nobel Peace prize, remains under house arrest in Rangoon. The junta, led by General Than Shwe, has been accused by numerous human rights groups and journalists of the murder and forced labour of ethnic minority civilians, and increasingly brutal treatment of political opponents.
According to the Democratic Voice of Burma website, Mr Thet Win Aung had been active in student politics since 1988.
He was Vice-General Secretary of the Basic Education Student Union (BESU) in 1989 and was imprisoned in 1991 for nine months because of his involvement with the banned group. He was allegedly tortured during while in custody.
Upon his release, he returned to activism by publishing leaflets and organising demonstrations, but was forced to go into hiding in 1994. He was later involved in anti-government demonstrations in 1996 and 1998, and also helped to organise student protests.
His funeral is expected to be held on Tuesday evening.
