On 22 June 1941, just hours after the commencement of Operation Barbarossa, Danish police arrested Martin Nielsen and around 300 other members of the Communist Party of Denmark (DKP). On the 20 August, he and 106 of the arrested men in Copenhagen were deported from Vestre Prison to the Horserød camp.[3] In December 1942, the Danish authorities handed him and his file over to the Gestapo for interrogation.[2][4]
Nielsen, Martin (1945). Mennesker jeg mødte paa min Vej (in Danish). Nyt Bogforlag. 312 pages.
Nielsen, Martin (1947). Rapport fra Stutthof [Report from Stutthof] (in Danish). Gyldendal. 170 pages.
Nielsen, Martin (1948). Undervejs mod livet. Erindringer fra Vestre Fængsel og Horserødlejren under Danmarks besættelse (in Danish). Gyldendal. 142 pages.
Nielsen, Martin (1949). Fængselsdage og fangenætter. Erindringer fra Vestre Fængsel og Horserødlejren under Danmarks besættelse (in Danish). Forlaget Tiden. 156 pages.