Before World War II, the population of Białystok (with over 91,000 inhabitants according to 1931 census) was 43 percent Jewish. There were two Jewish cinemas in the city, several Jewish dailies, sports clubs, prominent political parties and a Jewish library with over 10,000 books. Cultural life was booming. Białystok was overrun by the Wehrmacht on September 15, 1939, during the German invasion of Poland, and one week later handed over to the Red Army attacking from the East, in accordance with the German–Soviet Frontier Treaty. On November 1–2, 1939, the prewar Białystok Voivodeship along with over half of the Second Polish Republic were annexed by the Soviet Union, becoming part of the Belorussian Soviet Socialist Republic. During the Soviet occupation, Jewish companies and shops were closed, and Jewish social, educational, and political institutions were considered illegal. In addition many Jewish and Polish "capitalists" were deported by Soviet authorities to Siberia. Thousands of Jewish refugees flocked in from the German zone of occupied Poland. The city remained in Soviet hands until June 1941.