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A pedestrian suffered serious injuries after being knocked into the River Taff after a collision with a car.
Paramedics were called to Bridge Road in Llandaff, Cardiff, at about 16:50 BST on Sunday, along with an air ambulance and fire crews.
The injured man was taken to the University Hospital of Wales - police said he was in a stable condition.
South Wales Police said the 28-year-old driver of the Ford Focus involved in the crash had been arrested.
The road was closed in both directions for about five hours while the police carried out their initial investigations and recovered the vehicle.
South Wales Police clarified initial reports which suggested the injured man had been a cyclist.