A locking device for multiple-section telescoping tubes wherein a smaller tube is telescopically retractable into or extensible from within a larger tube includes a pair of opposing blocks, disposed within the telescoping tubes, one block being attached to the smaller tube, the other block being configured to move laterally with respect to the first block when the blocks are moved longitudinally with respect to each other. Depending on the direction of longitudinal movement of the blocks, one block may be caused to bear against an inner side of the larger tube to resist retraction of the smaller tube, or the blocks may be allowed to move away from the inner side of the larger tube to allow free sliding extension of the smaller tube. The device also includes means for selectively longitudinally moving one of the blocks with respect to the other, so as to allow free sliding retraction of the smaller tube, when desired.