The navigation activity is an every day practice for any human being capable of locomotion. Our objective in thiswork is to reproduce this crucial human activity inside virtual environments. Putting together the high complexityof a realistic environment such as a city, a big amount of virtual humans and the real-time constraint requires tooptimize each aspect of the animation process. In this paper, we present a suitable topological structuring of thegeometric environment to allow fast path finding as well as an efficient reactive navigation algorithm for virtualhumans evolving inside a crowd.