Far right activists bearing banners had gathered to hand out leaflets in the square when they were confronted by left wing extremists from Antifacist Aktion (AFA).
“They were throwing things at each other and were generally rowdy, but we managed to separate them,” said Björn Engström, Stockholm police spokesman.
The fighting began at around 2.30pm. AFA extremists congregated when they discovered where the right wingers were handing out their material.
Neither of the groups, each containing around 20 people, had permission to hold a demonstration. Nobody was injured in the brawl.