In 1905 the town was inhabited by about 7000 people of which 65.4% were Poles, 28.9% Germans and 5.5% Jews. In the surrounding county, Poles comprised 85.6% of the population. The hundred-year-long Prussian rule came to an end with the outbreak of the Wielkopolska Uprising in 1918, shortly after Poland regained independence, and in 1920, the town officially once again became part of Poland. About 800 local Poles formed the Września Volunteer Legion ("Legia Ochotnicza Wrzesińska") under the command of Stanisław Mycielski to fight against the Soviet invasion. It was around this time that construction of the district office building was completed. The 68th Infantry Regiment was stationed in Września in the interbellum.