In a modern and big city, the security system is not a single layered single module system. Instead, it consists of many layers with autonomous subsystems which are capable of interfacing reciprocally in an efficient and coordinated way horizontally and vertically. The design of such indigenous subsystems and ensuring tight interaction between them taking into account high usability and resource utilization is not a simple task. In this regard, this article provides a comprehensive overview of the security systems in smart cities. We begin by introducing trends and key stakeholders in smart cities. Next, we provide examples of mass surveillance systems in developed countries like USA, UK, and China. Finally, we highlight some of the major challenges in complete realization of smart cities. The security aspects discussed in this study can be beneficial in designing future smart cities.