Populating virtual environments (VEs) with large crowds is a subject that has been tackled for several years. Solutions have been proposed to offer realistic trajectories as well as interactivity, but limitations remain on the environment dimensions with respect to population density. In this paper, we extend the concept of motion patches [Lee et al. 2006] to densely populate large environments. We build a population from a set of blocks containing a pre-computed local crowd simulation. Each block is called a crowd patch. We address the problem of computing patches, assembling them to create VEs, and controlling their content to answer designers' needs. Our major contribution is to provide a drastic lowering of computation needs for simulating a virtual crowd at run-time. We can thus handle dense populations in large-scale environments with performances never reached so far. Our results illustrate the real-time population of a potentially infinite city with realistic and varied crowds interacting with each other and their environment. We discuss the advantages and drawbacks of the proposed solution, and its possible improvements in the future.