In this paper, we address the problem of privacy in video surveillance systems. More specifically, we consider the case of H.264/AVC which is the state-of-the-art in video coding. We assume that Regions of Interest (ROI), containing privacy-sensitive information, have been identified. The content of these regions are then concealed using scrambling. More specifically, we introduce two region-based scrambling techniques. The first one pseudo- randomly flips the sign of transform coefficients during encoding. The second one is performing a pseudo-random permutation of transform coefficients in a block. The Flexible Macroblock Ordering (FMO) mechanism of H.264/AVC is exploited to discriminate between the ROI which are scrambled and the background which remains clear. Experimental results show that both techniques are able to effectively hide private information in ROI, while the scene remains comprehensible. Furthermore, the loss in coding efficiency stays small, whereas the required additional computational complexity is negligible.