1. A surveillance system comprising at least one processor to implement: a foreground detection unit which predicts a foreground pixel based on foreground region information and motion information extracted from a video frame and performs validation on the predicted foreground pixel based on a texture feature so as to extract a foreground region; and an event detection unit which classifies the video frame based on the motion information, and determines whether an abnormal event has occurred in a monitoring region, based on results of classifications of a certain number of video frames, wherein the event detection unit comprises: an object classification unit which determines whether an object in the foreground region is a human being, based on information about a size and location of the foreground region, and classifies a foreground region determined to be a human being into a group of people or an individual person, based on information about a shape and location of the foreground region; a number-of-people prediction unit which predicts a number of people in the video frame based on a result of classification of the foreground region; a frame classifier which classifies the video frame based on a number of foreground regions, the number of people in the video frame, an average motion size of the foreground region, and a degree of consistency of motion directions in the foreground region; and an event determination unit which determines that an abnormal event has occurred in the monitoring region, if a ratio of a number of times an abnormal frame is classified among the certain number of video frames to a total number of times of the classifications is equal to and greater than a first threshold value.