The security region of a power system is an important and timely issue; different stability criteria may be limiting. Rotor-angle stability can be improved by modulating active power of installed high-voltage direct current (HVDC) links. This paper proposes a new centralized nonlinear control strategy for coordinating several point-to-point and multiterminal HVDC systems based on Lyapunov theory. The proposed control Lyapunov function is negative semi-definite along the trajectories and uses the internal node representation of the system. The proposed control Lyapunov function increases the domain of attraction and, thus, improves the rotor-angle stability. Nonlinear simulations are performed on the IEEE 10-machine 39-bus system which shows the effectiveness of the controller. In comparison, simulations using the conventional lead-lag controller are also run.