This paper introduces a novel sensor information fusion system enabling security and surveillance in large scale sensor saturated urban environments. The system is built over state-of-the art sensor networks middleware and provides information fusion at multiple layers. A distinguishing characteristic of the system is that it support seamless integration with semantic web middleware (including ontologies and inference mechanisms), which enable intelligent high-level accurate reasoning. This is a key functionality for efficient surveillance in large scale environment, where manual inspection of individual tracking systems becomes extremely resourceful and overall impractical. A proof-of-concept implementation of the system manifests its benefits and technical challenges, while also outlining lessons learnt.