Nicknamed "Magor," or "Looney," by friends, the trained art historian was the artistic director of the Plastic People of the Universe rock band, whose trial for disorderly conduct in 1976 led to the set-up of the Charter 77 dissident movement that gave prominence to future President Vaclav Havel.
Magor, known in the past decades for his long grey hair and wild partying as well as large body of poems, died of blood loss overnight, his friend Jan Machacek told Reuters. Another friend said Magor had suffered from internal bleeding before.
"For years he had been my good friend, who influenced social movements in our country in a major way," ex-president Havel, who himself was jailed under the Communists, said in a statement.
"I am glad he lived to see better times, for which he contributed a lot, and my thoughts are with him and his work."
Magor's approach to dealing with then-Czechoslovakia's oppressive government was simple but brave.
"The only issue was to be able to do one's own thing, music, art, totally independently of what was happening here on the official scene," he once said in a television interview.
"But because we lived in a society where the communists controlled the entire public life, any activity that was independent was a deadly threat to them."
Magor was first imprisoned in 1973-1974 after he literally ate parts of a Communist party newspaper, while also singing an anti-Russian song in a Prague pub. Russians were especially unpopular after the 1968 Soviet-led invasion of Czechoslovakia.
In total, he served eight-and-a-half years in some of the toughest Communist jails in the 1970s and 1980s.
His most famous works, published underground and later republished after the 1989 fall of Communism, include "Magorovy Labuti Pisne," or "Magor's Swan Songs," a collection he composed while in prison and had to memorize in its entirety because he was not allowed to write.
The Plastic People band was banned and its private concerts repeatedly busted by police, but their open disregard helped form a group of free-thinking intellectuals who embarked on building a life outside the system including private art exhibitions and an underground university.
The band later fell apart but was reformed in the late 1990s when it played at Havel's invitation at a reception at Prague Castle, and has toured frequently since.
The story of the Plastic People, and Havel, inspired British playwright Tom Stoppard, whose "Rock'n'Roll" play premiered at London's Royal Court Theater in 2006.
After the end of communist rule, Magor remained a public figure, often confronting the wrongs of a young democracy.
At a party last month, he looked frail and complained his condition did not allow him to drink, his frequent source of inspiration.
"Since doctors banned me from drinking, I have stopped writing," a friend quoted him as saying.