Related Rule
Practice Relating to Rule 106. Conditions for Prisoner-of-War Status
Section B. Levée en masse
Belgium’s Law of War Manual (1983) states:
The population of a non-occupied territory who spontaneously take up arms to resist the invading forces without having had time to form themselves into an organized resistance movement or to join the regular armed forces are considered combatants on the condition that this population:
a. respects the laws and customs of war;
b. carries arms openly. 
Belgium, Droit Pénal et Disciplinaire Militaire et Droit de la Guerre, Deuxième Partie, Droit de la Guerre, Ecole Royale Militaire, par J. Maes, Chargé de cours, Avocat-général près la Cour Militaire, D/1983/1187/029, 1983, p. 20.
In 1991, a Belgian parliamentary report considered that in the case of a levée en masse, actions in defence of the territory are permitted and justified by law even if they are not ordered by a proper authority. 
Belgium, Senate, Report, Enquête parlementaire sur l’existence en Belgique d’un réseau de renseignements clandestin international, 1990–1991 Session, Doc. 1117-4, 1 October 1991, § 24.