A document authentication method compares a target document image (scanned image) with an original document image at multiple levels, such as block (e.g. paragraph, graphics, image), line, word and character levels. The paragraph level comparison determines whether the target and original images have the same number of paragraphs and whether the paragraphs have the same sizes and locations; the line level comparison determines if the target and original images have the same number of lines and whether the lines have the same sizes and locations; etc. Document segmentation is performed on the target and original images to segment them into paragraph units, line units, etc. for purposes of the comparisons. The original document may be segmented beforehand and the segmentation information stored for later use. The authentication process may be designed to stop when alterations are detected at a higher level, so lower level comparisons are not carried out.