This paper presents a low complexity configurable semi-fragile watermarking scheme for content-based H.264/AVC authentication, which allows content-preserving manipulations such as video transcoding techniques, while it is very sensitive to content-changing and frame manipulations. A low cost spatial analysis is exploited to maximize robustness and lessen the incurred distortion. Invariant features are extracted from intra macroblocks, and frame index timing information constituting the content-based authentication codes are embedded/extracted into Quantized DCT (QDCT) coefficients in a Group of Pictures GOP-based fashion. The embedded watermark can be detected and verified by partially decoding intra and inter macroblocks without the prior knowledge of the original video or complete video decoding. Spatial and temporal tampering can be characterized and localized by comparing the extracted and re-generated feature-based watermarks. Experimental results demonstrate that the performance of the proposed scheme is indeed superb in terms of bit-rate and perceptual quality. Moreover, various types of attacks have been simulated and characterized.