Shortly before the outbreak of the Warsaw Uprising, Mazurkiewicz was made commander of the Radosław Group. This force was one of the largest, best trained and equipped Polish units in the uprising. After the initiation of the uprising, the unit seized major portions of the Wola suburbs, and subsequently defended it against German attacks carried out by troops under the command of SS Gruppenführer Heinz Reinefarth and Standartenführer Oskar Dirlewanger. One of the battalions of the group, Battalion Zośka, liberated the Gęsiówka concentration camp located within Warsaw and freed 384 prisoners (mainly Jews), most of whom then joined the unit. The Radosław Group fought its way to Warsaw Old Town borough when further defense in Wola became impossible. In the areas of Wola that Reinefarth's and Dirlewanger's troops recaptured from the insurgents, at least 40,000 civilians and prisoners of war were murdered in the Wola massacre. On 11 August he was seriously wounded during the fighting