After the Indonesian declaration of independence, from October 14, 1945 to November 23, 1945, parts of the fort were also used as a military base by the "Tentara Keamanan Rakjat" ("People's Security Army"), a predecessor of the Indonesian National Army. During which time, around 3,500 Dutch civilians were interned by Indonesian freedom fighters inside the Fort Willem I; the internment camp used former prisoners and guards for camp security.