In June, Maina Kiai, the special rapporteur at the United Nations on the rights to freedom of peaceful assembly and of association, cited Mr. Baek’s case in criticizing what he called an excessive use of water cannons by the police and shrinking space for exercising the right to peaceful assembly under Ms. Park.

“In footage made available to the special rapporteur, the water cannon was used against largely peaceful crowds. In certain cases, lone individuals were targeted, a use difficult to justify,” he wrote. “The case of Mr. Baek Nam-gi is a tragic illustration of this.”

Mr. Baek was born in 1947 in Boseong in rural southwestern South Korea. He was expelled from Chung-Ang University in Seoul in 1971 for leading a demonstration against Mr. Park’s deployment of troops on college campuses to intimidate student activists. He was later allowed to re-enroll but was soon on the run from the police for organizing protests against Mr. Park’s constitutional revision aimed at extending his dictatorship. While in hiding in a cathedral in Seoul, he became a Roman Catholic.

He was expelled again from school in 1975 but returned after the assassination of Mr. Park by his intelligence chief in 1979.

Mr. Baek then led students in a march against Chun Doo-hwan, an army major general who seized power in a coup after Mr. Park’s death. Mr. Chun’s martial-law troops arrested Mr. Baek during raids on school dormitories in 1980. He was expelled from school a third time and was later sentenced to two years in prison. He was freed in 1981.