Following April 1941 Nazi invasion and dismemberment of the Yugoslav Kingdom, Velebit stayed in Zagreb that became capital of the newly created Nazi client, Ustaše-run puppet state entity Independent State of Croatia (NDH). He operated as an underground collaborator of the KPJ-established People's Liberation Front. While working underground he used the alias name Vladimir Petrović, although due to being a well known and respected lawyer before the war he experienced no trouble with NDH authorities.