The man is reported to have been seriously assaulted by neo-Nazis taking part in the demonstration by the Swedish Resistance Movement (Svenska motståndsrörelsen - SMR)) on Stortorget in the centre of the town.
Eye witnesses have reported how demonstrators punched and kicked the man, as well as assaulting him in the stomach with their banners.
The tumult began when a woman spectator started to film the demonstration and one of the demonstrators seized her mobile phone. When a man entered the fray to help the woman, the assault was then directed towards him.
The police used pepper spray to break up the melee and a 35-year-old man was arrested on suspicion of aggravated assault.
On their homepage the Swedish Resistance Movement claimed the punch up was caused by "anti-fascists" who had gathered to disturb their demonstration.
The SMR themselves named the arrested man as Pär Sjögren and stated that several members of their group tried to prevent the arrest using violence, giving up when the police used "tear gas" against them.
The Swedish Resistance Movement is recognised as one of the key members of the white power movement in Sweden and their publication material openly praises the life of Adolf Hitler.