This paper presents a framework for controlling groups of autonomous mobile robots to achieve predetermined formations based on leader-following approach. A three-level hybrid control architecture is proposed to implement both centralized and decentralized cooperative control. Under such architecture, we decompose the global-level formation control problem of N robots into decentralized control problems between N-1 followers and their designated leader. In the leader-follower control level, two basic controllers are proposed to make the following robot keep relative position with respect to the leader and avoid collisions in the presence of obstacles. Then, graph theory is introduced to formalize specified formation patterns in a simple but effective way, and two types of switching between these formations are also proposed. Numerical simulations and physical robot experiments show the effectiveness of our approach