The girl was standing outside a school in the north west of the city when she was approached by a man who forced her to accompany him to City Hall in the city centre.
“They travelled using public transport rather than by car,” said police spokesman Mats Eriksson.
It is not yet clear what happened next. Two men who were sitting in a parked car by the quayside heard the girl crying for help at around 3pm. They helped her out of the water and called an ambulance.
The girl told the men that she had been pushed into the water. She was taken to hospital to be treated for hypothermia but did not sustain any serious injuries.
“We are treating the matter with the utmost seriousness and are now trying to find out more about what exactly happened. We are classifying the incident as a case of abduction and attempted murder,” said Mats Eriksson.
The girl is to be questioned more thoroughly about the incident on Wednesday, police said.