Demand response has become a key feature of the future smart grid. In addition to having advanced communication and computing infrastructures, a successful demand response program must respond to the needs of a power system. In other words, the efficiency and security of a power system dictate the locations, amounts and speeds of the load reductions of a demand response program. In this paper, we propose an event-driven emergency demand response scheme to prevent a power system from experiencing voltage collapse. A technique to design such a scheme is presented. This technique is able to provide key setting parameters such as the amount of demand reductions at various locations to arm the demand response infrastructure. The validity of the proposed technique has been verified by using several test power systems.