A toy construction system comprising toy construction elements (410, 412, 550, 810a-c, 1103), the toy construction elements comprising coupling means (102, 103, 105, 305, 405, 705) for releasably interconnecting the toy construction elements, the toy construction system further comprising: at least two marker construction elements (11, 311, 411, 511, 512, 513, 531, 532, 533, 541, 542, 543, 551, 552, 553, 711, 713, 715, 811, 1111a-b) comprising such coupling means and each having a visual appearance recognisable by an image processing means, where the two marker construction elements are releasably connectable with each other via their respective coupling means, so as to form a composite marker element; the at least two marker construction elements being configured, when added to the toy construction model, to transform the toy construction model constructed from said toy construction elements into an AR-enabled model which allows a data processing system to generate and superimpose computer-generated images responsive to a detected presence of the marker construction element; and a data processing system comprising image capturing means, image processing means, and display means, wherein the data processing system is adapted to: - capture an image of a toy construction model (10, 613) constructed from the toy construction elements and the at least two marker construction elements; - process the captured image to detect at least the composite marker element within the captured image; - responsive to the detected composite marker element generate a computer-generated image (14, 623); characterised in that generating the computer-generated image comprises triggering a behaviour of one or more virtual characters. at least two marker construction elements (11, 311, 411, 511, 512, 513, 531, 532, 533, 541, 542, 543, 551, 552, 553, 711, 713, 715, 811, 1111a-b) comprising such coupling means and each having a visual appearance recognisable by an image processing means, where the two marker construction elements are releasably connectable with each other via their respective coupling means, so as to form a composite marker element; the at least two marker construction elements being configured, when added to the toy construction model, to transform the toy construction model constructed from said toy construction elements into an AR-enabled model which allows a data processing system to generate and superimpose computer-generated images responsive to a detected presence of the marker construction element; and a data processing system comprising image capturing means, image processing means, and display means, wherein the data processing system is adapted to: - capture an image of a toy construction model (10, 613) constructed from the toy construction elements and the at least two marker construction elements; - process the captured image to detect at least the composite marker element within the captured image; - responsive to the detected composite marker element generate a computer-generated image (14, 623); characterised in that generating the computer-generated image comprises triggering a behaviour of one or more virtual characters. - capture an image of a toy construction model (10, 613) constructed from the toy construction elements and the at least two marker construction elements; - process the captured image to detect at least the composite marker element within the captured image; - responsive to the detected composite marker element generate a computer-generated image (14, 623); characterised in that generating the computer-generated image comprises triggering a behaviour of one or more virtual characters.