The activating mechanism comprises a manually-operated control lever rotatably constrained at a first end thereof to a pivot arranged in proximity of a front portion of a handle grip of a spray gun and an activating lever acting on a pusher element of an intercept valve. The activating lever has a first end portion which is rotatably constrained on a hinging pivot arranged in proximity of a back portion of the handle grip and a second end portion which is operatively constrained to the control lever.