WITNESS proposes an innovative framework for situational awareness and decision making, to improve the effectiveness of security forces in preventing and dealing with an urban attack. The pervasive and scalable nature of this framework, based on a sensor data fusion engine and scene understanding models, will facilitate the security forces to leverage the information gathered by multiple sensors (e.g. fixed visible/IR cameras, microphones, UAV payloads) and devices (iVest of policeman and connected police vehicles), since it will obviate the need for manual surveillance and processing of the sensor streams (e.g., watching hundreds of cameras).