Police spokesman Sven-Erik Olsson says two people were hospitalised and a policeman was injured in the back after being hit by a heavy object.
Olsson said around 200 people participated in the planned, peaceful demonstration in the suburb of Karrtorp today when they were attacked by a smaller group of about 40. Those detained are suspected of rioting and various assault charges.
The demonstration was organised by a local citizen group as a protest against increased neo-Nazi campaigning in the area. A neo-Nazi group called the Swedish Resistance Movement claimed responsibility for the attack on its website.