Hundreds of masked men marched through Stockholm's main train station on Friday evening, reportedly beating up refugees and anyone who didn't appear to be ethnically Swedish.
Wearing all-black balaclavas and armbands, the men "gathered with the purpose of attacking refugee children," Stockholm police spokesperson Towe Hagg said.
"I saw maybe three people who were beaten. That was no football brawl or something similar. They targeted migrants. I was quite scared and ran away," an eyewitness told the Aftonbladet newspaper.
Before the attacks, the mob handed out leaflets with the slogan "It is enough now!" which threatened to give "the North African street children who are roaming around" the "punishment they deserve".
The leaflet refers to the death of social worker Alexandra Mezher, who died after being stabbed at a refugee shelter for unaccompanied children.
Four people have been arrested in connection with the attacks - one for assaulting a police officer, while the others were charged with being masked in public, which is illegal in Sweden. All risk fines.
After the attack, the Swedish Resistance Movement, a neo-Nazi group, released a statement claiming the attack had "cleaned up criminal immigrants from North Africa that are housed in the area around the Central Station".
The statement added: "These criminal immigrants have robbed and molested Swedes for a long time."
"Police have clearly shown that they lack the means to stave off their rampage, and we now see no other alternative than to ourselves hand out the punishments they deserve."
Sweden received a record 160,000 refugees last year.
The country has seen a sharp decline in newcomers since photo ID checks were introduced this month.
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