A Cessna 525C Citation CJ4 aircraft, registration EC-MOQ, experienced a collapse of the left main gear during the landing run just as it was starting to vacate the runway via taxiway E3 at Santiago de Compostela Airport, Spain. The aircraft came to a stop inside the runway. The marshaller noticed that the left gear had collapsed and was outside its extension-retraction axis. Both engines were turned off, the aircraft was secured and the passengers were evacuated. Runway 17/35 at the Santiago airport remained closed for two hours and 15 minutes while the aircraft was raised and towed to its hangar. The occupants were not injured. The damage to the aircraft was limited to the left main landing gear leg and to part of the left wing, which was affected by deformations and the collapse of the left landing gear leg. A subsequent inspection showed that the bolt that attaches the aft trunnion pin to the trunnion was installed incorrectly, which allowed the trunnion pin to shift to the rear, decoupling from the trunnion and causing the leg to collapse. The bolt was also verified to be incorrectly installed in the right landing gear leg, although the trunnion pin did not come out of its fitting and the right leg remained in its position. The collapse of the left landing gear leg was caused by an error when installing the aft bolt into the trunnion pin, which allowed the trunnion pin to move in the trunnion fitting in the aft wing spar. The installation mistake in the main landing gear legs and the accumulation of other minor errors indicate that the maintenance facilities of the aircraft manufacturer, Textron Aviation, do not have standard installation (removal) procedures that are known to all of the personnel involved. Causes/Contributing factors: The collapse of the left landing gear leg was caused by the improper installation of the bolt in the trunnion pin, which allowed the pin to move inside the trunnion and the trunnion fitting in the aft wing spar.