In early 1905, Prystor became one of the leaders of the Combat Organization of the Polish Socialist Party. Using the pseudonym "Katajama", he organized groups of activists in Warsaw, and in March of that year, he carried out the assassination of Russian Police Officer Karl Nolken. Later on, he participated in a number of raids of the Combat Organization, including bank robberies, terrorist attacks on soldiers and police officers and acts of sabotage. After the split in the party, he joined Polish Socialist Party – Revolutionary Faction, and in 1906 left Congress Poland for Krakow, located at that time in Austrian Galicia. On June 18, 1906, Prystor married Janina Bakun, a fellow member of the Combat Organization.