Anchoring system for a guardrail post in a rigid bracket, said post comprising a front face and a back face, said system comprising: - a post base; - at least one plate (4, 4a, 4b) for anchoring to the rigid bracket; - anchoring means (8, 9, 10) of the at least one plate (4, 4a, 4b) for anchoring to the rigid bracket; wherein said post base is coupled mechanically with said at least one anchoring plate (4, 4a, 4b) through a mechanical coupling, said mechanical coupling being a rigid coupling which is deformable when rotated under the action of a force F applied on the post, having an horizontal component oriented from the front to the back of the post, said mechanical coupling getting plastically deformed when said horizontal component generates a force moment M greater than a first threshold value S1 on the anchoring system, characterized in that - said mechanical coupling is configured to get broken by torsion in fragile areas (5a, 5b) of the mechanical coupling when said horizontal component of the applied force generates a force moment greater than a second threshold value S2 on the anchoring system, the fragile areas (5a, 5b) being configured so that the post is completely decoupled from the at least one anchoring plate (4, 4a, 4b) after breakage of the fragile areas (5a, 5b), said anchoring means (8, 9, 10) of the at least one plate (4, 4a, 4b) for anchoring to the rigid bracket being configured to resist to a force moment equal to said second threshold value S2; - the at least one anchoring plate (4, 4a, 4b) comprises a through slot extending from the front to the back of said at least one anchoring plate (4, 4a, 4b) up to some depth without reaching the back edge thereof; - the at least one anchoring plate (4, 4a, 4b) comprises at least one through hole to receive the means (8, 9, 10) for anchoring to the rigid bracket, said through hole being connected to said slot. - a post base; - at least one plate (4, 4a, 4b) for anchoring to the rigid bracket; - anchoring means (8, 9, 10) of the at least one plate (4, 4a, 4b) for anchoring to the rigid bracket; wherein said post base is coupled mechanically with said at least one anchoring plate (4, 4a, 4b) through a mechanical coupling, said mechanical coupling being a rigid coupling which is deformable when rotated under the action of a force F applied on the post, having an horizontal component oriented from the front to the back of the post, said mechanical coupling getting plastically deformed when said horizontal component generates a force moment M greater than a first threshold value S1 on the anchoring system, - said mechanical coupling is configured to get broken by torsion in fragile areas (5a, 5b) of the mechanical coupling when said horizontal component of the applied force generates a force moment greater than a second threshold value S2 on the anchoring system, the fragile areas (5a, 5b) being configured so that the post is completely decoupled from the at least one anchoring plate (4, 4a, 4b) after breakage of the fragile areas (5a, 5b), said anchoring means (8, 9, 10) of the at least one plate (4, 4a, 4b) for anchoring to the rigid bracket being configured to resist to a force moment equal to said second threshold value S2; - the at least one anchoring plate (4, 4a, 4b) comprises a through slot extending from the front to the back of said at least one anchoring plate (4, 4a, 4b) up to some depth without reaching the back edge thereof; - the at least one anchoring plate (4, 4a, 4b) comprises at least one through hole to receive the means (8, 9, 10) for anchoring to the rigid bracket, said through hole being connected to said slot.