Dozens of masked men, thought to be members of neo-Nazi gangs, attacked migrants in Stockholm on Friday night amid rising tension over immigration, Swedish police said.
Video footage showed a number of men dressed in black rampaging in and around the city's main train station, followed by officers.
Anti-riot police and helicopters were deployed around the centre after hearing that extremists were planning "aggression on unaccompanied migrant minors".
One witness said: "I was passing by and saw a masked group dressed in black ... start hitting foreigners. I saw three people molested."
Up to 100 people with their faces covered had gathered on the Sergels Torg pedestrian square, a popular meeting point for young people, including unaccompanied migrants.
Police said four masked men were questioned after they handed out anti-migrant leaflets urging Swedish citizens to organize illegal protests against suspected refugees.
Around "100 hooligans" from the AIK and Djurgarden football clubs had gathered on Friday in order to "sort out the criminals coming in from North Africa", an online forum, citing sources, reported.
Sweden has been one of the top destinations for asylum seekers coming to Europe, receiving a record 160,000 arrivals last year.