I'm not sure about the Japanese Mainland, but in China foreigners were free to observe in both the Chinese and Japanese zones of control until Peal Harbor and the invasion of SE Asia in 1941/42. After the declaration of war the foreigners in China were also put in internment camps. One of the best books on the subject is Shantung Compound which details how a group of international foreigners came together to build a local government for their internment camp, and details the many problems and trials they went through. The lack of food, adequate housing, policing, and daycare created many interesting issues that the community had to come together to solve.