Soe Win, a reputedly ruthless member of the ruling military junta, had been blamed with overseeing a 2003 attack against democracy icon Aung San Suu Kyi.
The fourth-ranking member of the junta, he had been ailing for months with what relatives said was acute leukemia. He returned Sept. 30 from extended hospitalization in Singapore and had been warded at Mingaladon Military Hospital on the outskirts of northern Yangon, relatives said.
Soe Win's death came as the junta continued its crackdown on democracy advocates that followed more than a month of street protests in the tightly controlled country.
His departure was unlikely to cause a ripple in the regime's grip on power. Soe Win had little if any policy-making role as prime minister and was largely considered a figurehead for the junta.
Lt. Gen. Thein Sein, who has been serving as acting prime minister at least since May, was expected to succeed Soe Win. Thein Sein is known as a fierce loyalist of Senior Gen. Than Shwe, the junta leader.
Soe Win was nicknamed the "Butcher of Depayin" for his role in the 2003 attack on Suu Kyi and her followers in the northern town of Depayin.
Details of the attack remain murky, but several dozen of Suu Kyi's supporters were believed killed when a mob of government supporters ambushed her motorcade. Soe Win is considered the mastermind behind the attack, according to diplomats, rights groups and government critics.
He first achieved notoriety as one of the officers who brutally suppressed a 1988 pro-democracy uprising, commanding troops around Rangoon University - a center for demonstrations - and giving orders to open fire on a crowd of protesters in front of Rangoon General Hospital. Rangoon is the old name for the Myanmar's former capital, Yangon.
Soe Win was also an air force chief and a commander for the northwestern military region of Myanmar. He joined the junta's inner circle as Secretary-2 in February 2003, and was promoted to Secretary-1 in an August 2003 Cabinet shake-up, replacing Khin Nyunt.
He is survived by his wife, and their son and daughter. Soe Win's twin brother died on Sept. 19.
The current junta seized power in 1988, then held a general election in 1990 but refused to recognize a landslide victory by the opposition party of Suu Kyi, who won the Nobel Peace Prize in 1991.
Simmering anger with the military's 45-year rule exploded in mid-August after it hiked fuel prices by as much as 500 percent _ a crushing burden in this impoverished nation. The marches soon ballooned into mass pro-democracy demonstrations led by the nation's revered Buddhist monks.
The military crushed the protests on Sept. 26 and 27. The government says 10 people were killed in the violence. But dissident groups put the death toll at up to 200. They say 6,000 people were detained, and that arrests continue.