Abstract: A security system for controlling access to a room or part of a building through an opening comprises a revolving door turnable only in a single predetermined direction of rotation about a vertical axis situated at the center of a passage defined by a pair of opposite walls each in the form of cylindrical sectors disposed at either side of the opening, and locking means for stopping the revolving door and locking it in a predetermined angular position in such a way as to lock in a person passing through the said passage if, for example, a weapons detector senses that he is armed. The security system is further provided with a curved sliding door angularly displaceable about the axis of rotation of the said revolving door between an open position adjacent one of the two passage-defining walls, and a closed position spanning the said two walls across the opening leading from the said passage into the room or part of the building into which access is controlled by the system.