In April 1940, Nazi Germany invaded and occupied Norway, and the regular political authorities were put out of the running. Johan Cappelen, County Governor of Sør-Trøndelag and later resistance member, set up the so-called Central Committee of Trondheim to administer the city, headed by Claussen together with Cappelen and Ivar Skjånes. The Central Committee was disbanded in the autumn by the Nazi authorities. Claussen continued as a barrister, but also became involved in resistance work. He was arrested on 3 August 1942 for "anti-German sentiments", and was incarcerated at Grini two days later, and released six days before the liberation of Norway on 8 May 1945.