1. A method for detecting persons, the method comprising: initially detecting the persons in a height-top-view including the persons to be detected; dividing the height-top-view into one or more regions, and estimating crowd density in each of the regions; determining, based on the crowd density, visible regions of the initially detected persons in each of the regions; for each of the initially detected persons, extracting a first gradient feature of the person from the height-top-view, and a second gradient feature of the person from a grayscale image or a color image corresponding to the height-top-view, respectively; for each of the initially detected persons, determining, based on the extracted first gradient feature and second gradient feature, using a previously constructed classifier corresponding to the determined visible region of the person, a confidence level of the initially detected person; and correcting, based on the confidence level, a detection result of the initially detected persons.