Polish students did not enjoy their freedom for long. Following the martial law in Poland, the Association was banned on February 5, 1981, and many of its activists were arrested. The NZS was still active in the underground, especially in large population centers, such as Warsaw, Wrocław and Kraków. Some of its members, such as Teodor Klincewicz from Warsaw, were actively involved in various forms of anti-government protests. Also, students of the Law Department at Łódź University, in opposition to martial law, organized a sit-in, which was brutally broken by the riot police. Another strike was broken at Kraków's Pedagogical University. At some point in the mid-1980s, the Association began cooperation with high-school students organization Federation of Fighting Youth.