Petar Dertliev, a former dissident who revived the Social Democratic Party after a decades-long ban by Bulgaria's communist regime, has died aged 84.

Dertliev was one of the founders and leaders of the anti-communist Union of Democratic Forces and member of the first freely elected post-communist parliament in 1990.

Born in 1916, Dertliev graduated from the University of Sofia's medical faculty and became an assistant professor there in 1943. During the 1930s, he was arrested several times because of his social democratic activities. In 1948, after communists had taken power in the country, he was arrested and given 10 years in prison for his political beliefs. After the fall of communism in 1989 he revived the Social Democratic Party and was its leader until his death.

''An absolute idealist is gone,'' President Petar Stoyanov said. ''His youth was crushed by the communist labour camps, but no-one could crush his spirit.''

Dertliev is survived by his wife and two daughters.