This paper presents a service-oriented platform for performing efficient and scalable on-line power system security analysis. This analysis is a complex, large-scale, and compute-intensive problem whose solution requires different distributed resources and functionalities. These can be both high-level services provided by organizations belonging to different functional areas of electric industry and low-level services provided by service providers on the Internet. The platform described in the paper is able to integrate services of different kinds, such as real-time data acquisition from several sources, high computational power and data storage capabilities, to perform complex computations on large scale distributed systems. The paper discusses a Web Services-based implementation of the platform, which is centered on a BPEL-modeled workflow enactor able to exploit a Grid computing middleware to execute compute intensive code.