An object loading and unloading system for loading and unloading objects in a graphically simulated virtual environment, or virtual world, is described. A world object manager manages multiple object loaders to load and unload objects as geographic cells move within and out of a loading radius of each loader based on a position of a camera or view within the virtual world. Each object loader has an associated priority level, and the world object manager notifies a loader to load its corresponding objects in a cell only after all loaders having higher priority levels, which also have that cell in their respective loading radii, have already loaded their corresponding objects in that cell. The world object manager may expose various programming interfaces to provide an extensible object loading system whereby third parties can define new loaders to be incorporated into the hierarchical spatial object loading system.