By Nga Pham
BBC News, Hanoi
One of Vietnam's best-known dissidents, Hoang Minh Chinh, has died at the age of 85 after a long illness.
Chinh was one of the last of the older generation of activists
His family said he passed away peacefully on the first day of the Lunar New Year.
Hoang Minh Chinh was once a leading figure in the ruling Communist Party, holding several senior positions.
But he became disillusioned with the communist ideology, and began calling for more democracy. He spent many years in jail and under house arrest.
Born in 1922 in Nam Ha province, he joined the revolution in 1937.
He held a number of high positions in the Vietnamese government, including vice-minister of education, vice-director of the Nguyen Ai Quoc Communist Party School and director of the Marxist-Leninist Philosophy Institute.
He was considered one of the great ideologists of the regime until the 1960s, when he began criticising some of the ruling party's decisions.
He was jailed for being a member of the so-called "anti-party revisionist campaign".
During the last decade of his life, Mr Chinh was actively involved in the democratic movement in Vietnam, and restarted the Vietnam Democratic Party two years ago.
In 2005, in an unprecedented move, the Vietnamese government allowed him to go to the United States to receive treatment for pancreatic cancer.
During the trip, he appeared before a congressional committee to speak about the situation inside Vietnam, and called for greater pressure on the Vietnamese government.
The speech prompted an aggressive campaign against him in the Vietnamese media, but Mr Chinh was still allowed back into the country.
In 2006 he took part in the establishment of the largest democratic campaign to date, Bloc 8406.
But his past attachment to the communist ideology somewhat alienated him from the younger generation of activists, who wanted a clean break from the past.
Mr Chinh's health worsened last year.
He was suffering from numerous illnesses and was in and out of hospital for most of the year.