This paper addresses the optical-based 3D displacement measurement system applied in the structural dynamic observation of the 25th of April long-span suspension bridge in Lisbon (Portugal), namely, in its main span central section. The lack of nearby absolute reference points and the structure's dynamic behaviour do not allow applying conventional solutions. Being a metallic suspension bridge, other recently developed non-contact measurement solutions, such as global navigation satellite systems and microwave interferometric systems are not recommended since their measurement accuracy is affected by the multi-path effect resulting from reflections of electromagnetic waves in the bridge's structural components. The optical system comprised a high focal length digital camera, rigidly connected to the lower surface of the stiffness beam and orientated towards a tower foundation, where a set of four active targets was installed. Based on the pinhole camera model and being the camera's intrinsic parameters and targets world coordinates previously quantified, the targets image coordinates allowed determining the camera's projection centre world coordinates which, in the adopted measurement configuration, are considered representative of the central section spatial position, making possible its 3D displacement measurement. This paper describes the experimental procedures and results obtained in the camera's intrinsic parameterization and also in the field calibration test of the optical system. Displacement measurements related to a train passage in the mentioned suspension bridge are also shown and analysed. It was concluded that the described optical system is suitable for vertical and transversal displacement bridge observation within the determined measurement uncertainty.