In this paper, we propose a fast watermarking system that works on the H.264/AVC motion vectors. By restricting access to DCT coefficients and pixel information, the computational complexity of the watermark embedder/extractor is kept low and much lower than that of the H.264 decoder. The error propagation due to motion prediction compensation is monitored and its effect is limited by a tracking method that is based solely on the motion information from the bitstream. Although this work focuses on the H.264/AVC standard, the novel watermarking technique is also applicable to the MPEG1-2 and MPEG4 video standards