The suspected perpetrator was arrested, police said.
The Nazi group Nordic Resistance Movement (Nordiska motståndsrörelsen, NMR) gathered at Östermalmstorg and then Norrmalmstorg, attracting around 40 supporters. When the group later moved to Medborgarplatsen, it was met by opponents and a tense stand-off occurred.
“There was unrest and a police officer was assaulted while trying to keep the groups apart,” police spokesman Sven-Erik Olsson said.
It was not immediately apparent which side of the conflict the arrestee supported.
The Nazi group did not have a license for its demonstration, which was the latest in a string of appearances in Swedish cities, including repeated demos in Gothenburg.
The NMR was established in 1997 and promotes an openly racist and anti-Semitic doctrine.