Safety device to prevent the sudden closing of a movable element such as a closing element of a piece of furniture or a door, comprising a fixed support (13, 113, 213) and - a lever mechanism (20, 120, 220) comprising in turn at least an actuation lever (11, 111, 211) and a connection lever (18, 118, 218) both being connected at one of its ends to said fixed support (13, 113, 213), said connection lever (18, 118, 218) being connected in an articulated manner to said actuation lever (11, 111, 211), said actuation lever (11, 111, 211) being pivoted on a pin (12, 112, 215) and provided to be actuated by said movable element in order to oscillate with respect to said pin (12, 112, 215), between an open position and closed position; characterized in that it further comprises: - a first elastic member (25, 125, 225) which constantly applies a first elastic force on said lever mechanism (20, 120, 220) in order to constantly thrust said actuation lever (11, 111, 211) toward said open position; - a sliding element (21, 121, 221) provided to slide in a guide (23, 123, 223) made in said fixed support (13, 113, 213) between an initial position, in which said sliding element (21, 121, 221) finds itself when said actuation lever (11, 111, 211) is in said open position, and a final position in which said sliding element (21, 121, 221) finds itself when said actuation lever (11, 111, 211) is in said closed position; said sliding element (21, 121, 221) being mounted at said end of said connection lever (18, 118, 218) or of said actuation lever (11, 111, 211), and being connected to said fixed support (13, 113,213); - a second elastic member (30, 130, 230) which constantly applies a second elastic force, less than said first elastic force, on said lever mechanism (20, 120, 220) and provided to thrust said lever mechanism (20, 120, 220) toward an operating condition, in which said sliding element (21, 121, 221) is free to slide in said guide (23, 123, 223) between said initial position and said final position; said first elastic member (25, 125, 225) and said second elastic member (30, 130, 230) being fixed on one of its ends to said fixed support (13, 113, 213); - a clamping member (26; 128, 129; 226) provided to cooperate with said lever mechanism (20, 120, 220) to prevent said sliding element (21, 121, 221) from moving from said initial position toward said final position inside said guide (23, 123, 223), when a determinate impulsive-type actuation force exceeding a determinate value, greater at least than that of said second elastic force, is applied suddenly by said movable element on said actuation lever (11, 111, 121). - a lever mechanism (20, 120, 220) comprising in turn at least an actuation lever (11, 111, 211) and a connection lever (18, 118, 218) both being connected at one of its ends to said fixed support (13, 113, 213), said connection lever (18, 118, 218) being connected in an articulated manner to said actuation lever (11, 111, 211), said actuation lever (11, 111, 211) being pivoted on a pin (12, 112, 215) and provided to be actuated by said movable element in order to oscillate with respect to said pin (12, 112, 215), between an open position and closed position; - a first elastic member (25, 125, 225) which constantly applies a first elastic force on said lever mechanism (20, 120, 220) in order to constantly thrust said actuation lever (11, 111, 211) toward said open position; - a sliding element (21, 121, 221) provided to slide in a guide (23, 123, 223) made in said fixed support (13, 113, 213) between an initial position, in which said sliding element (21, 121, 221) finds itself when said actuation lever (11, 111, 211) is in said open position, and a final position in which said sliding element (21, 121, 221) finds itself when said actuation lever (11, 111, 211) is in said closed position; said sliding element (21, 121, 221) being mounted at said end of said connection lever (18, 118, 218) or of said actuation lever (11, 111, 211), and being connected to said fixed support (13, 113,213); - a second elastic member (30, 130, 230) which constantly applies a second elastic force, less than said first elastic force, on said lever mechanism (20, 120, 220) and provided to thrust said lever mechanism (20, 120, 220) toward an operating condition, in which said sliding element (21, 121, 221) is free to slide in said guide (23, 123, 223) between said initial position and said final position; said first elastic member (25, 125, 225) and said second elastic member (30, 130, 230) being fixed on one of its ends to said fixed support (13, 113, 213); - a clamping member (26; 128, 129; 226) provided to cooperate with said lever mechanism (20, 120, 220) to prevent said sliding element (21, 121, 221) from moving from said initial position toward said final position inside said guide (23, 123, 223), when a determinate impulsive-type actuation force exceeding a determinate value, greater at least than that of said second elastic force, is applied suddenly by said movable element on said actuation lever (11, 111, 121).