There's a very good argument that, after being abolished in 1861, serfdom was re-introduced in the early 1930s via collectivization. Instead of private ownership of serfs, the state effectively owned the peasants who were bound to their collective farms by a system of meagre food-for-labour payments, needing permission to leave the farm/village for any reason, etc. The peasants that "came with" the collective farms were only issued internal passports in the late 1960s. Anyone who knows anything about life in the USSR understands the implications of having no internal passport. You didn't quite need one to breathe when outside your city/town/village, but just about.

Without an internal passport a person would be detained at the first train station and, in best case, returned back to the farm.