Chart of Second Polish Republic population in largest cities (1930s)

By 1931, Poland had the second largest Jewish population in the world, with one-fifth of all the world's Jews residing within its borders (approx. 3,136,000). The urban population of interbellum Poland was rising steadily; in 1921, only 24% of Poles lived in the cities, in the late 1930s, that proportion grew to 30%. In more than a decade, the population of Warsaw grew by 200,000, Łódź by 150,000, and Poznań – by 100,000. This was due not only to internal migration, but also to an extremely high birth rate.

Reference 1: Population.

Reference 2: Warsaw, Łódź, Lwów, Poznań, Cracow, Wilno, Bydgoszcz, Częstochowa, Katowice, Sosnowiec, Lublin, Gdynia, Chorzów, Białystok, Kalisz, Radom, Toruń, Stanisławów, Kielce, Włocławek, Grudziądz, Brześć nad Bugiem, Piotrków Trybunalski and Przemyśl.

Source: http://en.wikipedia.org/wiki/Second_Polish_Republic