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.
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."
Police said four masked men were questioned after they handed out anti-migrant leaflets urging Swedish citizens to organize illegal protests against suspected refugees.
Sweden has been one of the top destinations for asylum seekers coming to Europe, receiving a record 160,000 arrivals last year.