According to Swedish police, one member of the organisation, which has links to far-right groups, was taken to hospital after some type of fluid was splashed into their eye.
Others filed reports to the police claiming they had been punched and kicked.
“We really do not know the origin of the quarrel, but those who have been victims of punches and kicks and have filed these reports all say that they belong to the Soldiers of Odin organisation, police spokesman Christer Fuxborg told TT newswire.
It remained unclear whether the brawl had taken place during one of the group’s night patrols, which they claim are necessary to protect Swedish citizens, particularly women, from violent attacks and sexual assaults carried out by immigrants.
Another Soldiers of Odin patrol took place in Uppsala on Saturday night, without causing any problems.
“There were about a dozen of them who came here… which of itself probably was not a major problem,” said Nina Dahlström, duty officer with the Uppsala police. “It could have been a problem if we had had 20 or 30 from the left wing who had come along and wanted a fight.”
Uppsala police had contacted left-wing groups asking them not to mount counter-demonstrations, she said.
Soldiers of Odin, founded by Finnish Nazi Mika Ranta, has previously conducted patrols in Borlänge, Trelleborg and Stockholm.
Several of its Swedish members have been convicted of serious crimes and Nazi sympathies, according to an investigation by Expo magazine.