Mr. Modzelewski graduated with a degree in history from the University of Warsaw in 1959. He was an assistant professor there, with interruptions for prison, until 1971. Forced by the authorities later to leave Warsaw, he moved to Wroclaw, where he continued his research at the University of Wroclaw until 1980, when he joined Solidarity.

He is survived by his wife, Malgorzata Goetz, and his daughter, Ewa.

Mr. Modzelewski’s fighting spirit was evident from an early age. In 1956, when he was 18, Polish workers rose up against poor economic conditions, prompting the Soviets to threaten invasion to squash the resistance. When Soviet forces were on their way to Warsaw, Mr. Modzelewski joined laborers in a car factory in filling bottles with gasoline to be thrown at advancing tanks.

It never came to that, however; Moscow eventually pulled back.

Mr. Modzelewski came to the attention of the authorities in 1964 after writing, with a fellow dissident, Jacek Kuron, “An Open Letter to Members of the Polish United Workers Party.” It was a scathing indictment of the Communist Party and its elitist practices, which the men called a betrayal of the ideals of Communism.

They called for a revolution to bring about a true workers’ democracy.

Mr. Modzelewski and Mr. Kuron were both expelled from the party and sentenced to three and a half years in prison.

Soon after leaving jail, Mr. Modzelewski helped ignite student protests at the University of Warsaw in March 1968, when he called for an end to censorship. He was sent back to prison.

The open letter and student protests led to a democratic opposition movement that coalesced in 1980 around Solidarity, the first independent trade union in Communist Poland.

In the end, Mr. Modzelewski felt the government of 1989 had betrayed the ideals on which it was founded, Irena Lasota, a friend and co-founder of the Institute for Democracy in Eastern Europe, based in Washington, said in a telephone interview.