This paper deals with the detection of moving objects in videos, directly from MPEG coded data, with a view to content-based video indexing. The detection is stated as a Markovian labeling issue in terms of macroblocks conforming or not to the estimated dominant image motion assumed to be due to the camera motion. The dominant motion estimation and the moving object detection stages only utilize MPEG motion vectors and DC coefficients of the discrete cosine transform (DCT) directly extracted from the MPEG bit stream of the processed video. Therefore, our method implies a very low computational cost. Experimental results have demonstrated the interest of the proposed approach.