Significant developments have taken place over the past few years in the area of vehicular communication (VC) systems. Now, it is well understood in the community that security and protection of private user information are a prerequisite for the deployment of the technology. This is so, precisely because the benefits of VC systems, with the mission to enhance transportation safety and efficiency, are at stake. Without the integration of strong and practical security and privacy enhancing mechanisms, VC systems could be disrupted or disabled, even by relatively unsophisticated attackers. We address this problem within the SeVeCom project, having developed a security architecture that provides a comprehensive and practical solution. We present our results in a set of two papers in this issue. In this first one, we analyze threats and types of adversaries, we identify security and privacy requirements, and we present a spectrum of mechanisms to secure VC systems. We provide a solution that can be quickly adopted and deployed. In the second paper, we present our progress towards the implementation of our architecture and results on the performance of the secure VC system, along with a discussion of upcoming research challenges and our related current results.