1. A system for surveillance of distributed geographical sites for a plurality of event, each event associated with at least one predefined characteristics and accessible to a plurality of event subscribers, the system comprises: a plurality of sensing device flexibly coupled to a plurality of geographically distributed site; a sensory information processing unit electrically coupled with the each sensing device and a combination thereof, the sensory information processing unit comprising: a video analytics module configured to identify and index the events in the captured sensory information and generate a cumulative metadata therefor; a plurality of participatory surveillance tool comprising mobile data gathering and communication devices, each participatory tool being selectively identified based on the event metadata and configured to share gathered sensory information corresponding to the said event, wherein the plurality of participatory surveillance tool comprises a proactive and an event triggered surveillance information input applications installed thereon the sensory devices, social network posts referring to a geo-tagged social network data, video and sound recordings received from mobile devices and text/voice call recordings from the plurality of local sites; a backend analytics server, coupled with a messaging middleware and a central storage, is configured to perform a multimodality filtration surveillance for anomaly detection in the events; and a search module adapted for a real threat detection comprising a temporal search for identification of a distinct spacio-temporal character the each event and a geo-spatial search providing a color graded heat-map illustrating geographical distribution of potential security events along with timestamp.