ROME—Marco Pannella, a maverick radical politician who was crucial to Italian postwar campaigns to legalize abortion, divorce and other social change, died Thursday. He was 86.

Mr. Pannella’s death was announced by Radio Radicale, the radio station of his political party. Known for his hunger strikes, sit-ins, and recourse to political referendums to push his liberal, oftentimes anti-church agenda, Mr. Pannella had been hospitalized at a Rome clinic in recent days.

Premier Matteo Renzi eulogized the grizzled veteran politician as a “lion of freedom,” and tributes poured in from across Italy’s political spectrum.

Mr. Pannella was one of the founders of Italy’s Radical Party in the 1950s. As a member of parliament and outside agitator over the ensuing decades, he was crucial to pushing the overwhelmingly Catholic Italy to legalize divorce and abortion.

On the international stage, the cigar-chomping activist was also a known peacenik, friends with the Dalai Lama and a fan of Martin Luther King, Jr. Despite his anticlerical rhetoric, he found common cause with the Vatican on efforts to end world hunger.