Modular construction system, comprising at least two construction elements (20; 30; 40; 50; 60; 70) and a coupling element (1; 10) which can be connected releasably to the construction elements, wherein: the coupling element (1; 10) comprises a plate-like body (2; 11) with two flat surfaces (3a, 3b) which are each provided with coupling means, wherein the coupling means of at least one of the two surfaces (3a, 3b) comprise a ring of resiliently pivotable lips extending at least substantially perpendicularly of the surface and provided with locking protrusions close to a first end zone; at least one of the two construction elements (20; 30; 40; 50; 60; 70) comprises at least one surface with at least one opening which is adapted to receive the lips of the coupling element (1; 10) in order to connect the coupling element (1; 10) and the construction element (20; 30; 40; 50; 60; 70), this opening being smaller than the distance between two opposite protrusions of the lips such that in a connected situation the protrusions of the lips of the coupling element (1; 10) engage on the surface of the construction element (20; 30; 40; 50; 60; 70) close to the opening; and the lips are adapted to displace inward at least close to their first end zones subject to a force exerted on the coupling element (1; 10) such that the distance between the two opposite protrusions of the lips can decrease to less than the size of the opening in the construction element (20; 30; 40; 50; 60; 70) in order to release the connection between the coupling element (1; 10) and the construction element (20; 30; 40; 50; 60; 70), characterized in that the plate-like body (2; 11) of the coupling element (1; 10) comprises an opening which extends between the two flat surfaces (3a, 3b), this opening of the plate-like body (2; 11) being provided with resiliently pivotable tongues extending radially inward which are connected to the lips, wherein the tongues of the opening of the plate-like body (2; 11) are displaced out of the plane of the plate-like body (2; 11) at least close to a first outer end of the tongues by a force being exerted thereon, such that due to displacement of the tongues out of the plane of the plate-like body (2; 11) the lips are displaced inward. the coupling element (1; 10) comprises a plate-like body (2; 11) with two flat surfaces (3a, 3b) which are each provided with coupling means, wherein the coupling means of at least one of the two surfaces (3a, 3b) comprise a ring of resiliently pivotable lips extending at least substantially perpendicularly of the surface and provided with locking protrusions close to a first end zone; at least one of the two construction elements (20; 30; 40; 50; 60; 70) comprises at least one surface with at least one opening which is adapted to receive the lips of the coupling element (1; 10) in order to connect the coupling element (1; 10) and the construction element (20; 30; 40; 50; 60; 70), this opening being smaller than the distance between two opposite protrusions of the lips such that in a connected situation the protrusions of the lips of the coupling element (1; 10) engage on the surface of the construction element (20; 30; 40; 50; 60; 70) close to the opening; and the lips are adapted to displace inward at least close to their first end zones subject to a force exerted on the coupling element (1; 10) such that the distance between the two opposite protrusions of the lips can decrease to less than the size of the opening in the construction element (20; 30; 40; 50; 60; 70) in order to release the connection between the coupling element (1; 10) and the construction element (20; 30; 40; 50; 60; 70), characterized in that the plate-like body (2; 11) of the coupling element (1; 10) comprises an opening which extends between the two flat surfaces (3a, 3b), this opening of the plate-like body (2; 11) being provided with resiliently pivotable tongues extending radially inward which are connected to the lips, wherein the tongues of the opening of the plate-like body (2; 11) are displaced out of the plane of the plate-like body (2; 11) at least close to a first outer end of the tongues by a force being exerted thereon, such that due to displacement of the tongues out of the plane of the plate-like body (2; 11) the lips are displaced inward.