Wireless communications technologies play a critical role for the Public Protection and Disaster Relief (PPDR) operational needs in crisis situations. The role of spectrum sharing is recently gaining momentum to address the major limitations of emergency communications including the lack of network capacity and interoperability issues. Cognitive radio is a critical enabler for the realization of the spectrum sharing models. In this paper we address the problem of transmit power control in secondary spectrum sharing. We consider the model where the Primary User (PU) imposes the interference temperature restriction and that interference temperature cannot be exceeded by transmitting Secondary Users (SU). Power control of the SUs is designed and implemented using a game theoretic approach. In order to fulfill the game we propose a utility function that maximizes Signal to Interferenceplus-Noise Ratio (SINR) and minimizes the transmit power of a SU. We then compare the performance obtained using our game theory based approach with that of a resource optimization approach. We observe that our approach is more sensitive to the transmission power levels and is cheaper in terms of computation costs.