Some time in the early 20th century, Prystor joined Polish Socialist Party, and became a close associate of Jozef Pilsudski. In September 1903, he left Vilna, and went to Switzerland, to undergo military training, together with Pilsudski and other activists. In 1904 Prystor, together with Jozef Kwiatek, Walery Slawek and Boleslaw Jedrzejowski, organized public protests against the forcible draft of ethnic Poles into the Russian Army to fight in the Russo-Japanese War. Also, he constructed bombs which damaged the monument of Tsar Alexander III of Russia.