Jan Jozef Lipski, leader of Poland's small Socialist Party and one of the country's best-known anti-Communist dissidents, died Tuesday of heart disease, the PAP news agency said.
Lipski, a writer, political activist and historian, died in a Krakow hospital at the age of 65.
Lipski was a co-founder in 1976 of the Workers' Defense Committee, which paved the way for the creation of Solidarity, the Communist bloc's first independent trade union, in 1980.
He joined Solidarity and was one of its most eminent figures, spending two years in jail after the Communist regime introduced martial law in 1981 in a bid to crush the union.
He was briefly released in 1982 and allowed to travel to London to have a heart pacemaker implanted.
A historian by profession, he helped re-create the prewar Polish Socialist Party in 1987 and remained its chairman until his death, although the party never regained its prewar prominence.