It was just before 3 pm on Tuesday that police were alerted to the hospital area in Falun.
One person had seen a man lift a child into a car and then hit the child in the face.
The man then locked the child in the car and left the scene, despite the sun shining and warming the car, according to police.
Police found the man in a nearby store.
He was allowed to accompany the car and drop the child off.
The man was immediately detained.
The child was examined by ambulance personnel and then left to a relative who came to the scene.
During the evening, prosecutors have decided to arrest the man suspected of assault.
The social services have been informed of what happened.