 In 1941, she participated in the first groups of the communist resistance led by Raymond Losserand and installed a small, rudimentary laboratory in her two-room apartment on the Place du Danube located in the 19th arrondissement in Paris.Taking the name Claudia in hiding, she worked with Colonel Dumont making grenades and detonators used in attacks organized by the youth resistance (called the Young Battalions) at the end of August 1941.Bloch was arrested by the French police on 16 May 1942.