The rank-size distribution in a system of settlements is considered as a multifractal phenomenon. The occurrence and temporal stability of the rank-size distribution is obtained as the result of a dynamic self-organization process in the nested system of settlements. The inherent time scale of this self-organization process and the obtained hierarchical structure are related to low intersettlement mobility of urban population, a tendency of the population to agglomerate, as well as distance and preference effects. A well-known migration model is used with three different approaches for the attractivity of a settlement for comparative purposes.