According to a statement released by police in the northern German city, the group wearing black clothes and hoods carried out the attack, the Guardian reported.
The European head of the social networking giant is under probe in Germany over Facebook's alleged failure to remove racist hate speeches.
German leaders and other eminent personalities are concerned about the rise in racial comments on Facebook as the nation fights to cope with the flow of refugees, the report added.
According to a Facebook spokesperson, "we can say that the allegations lack merit and there has been no violation of German law by Facebook or its employees."
The company added that it would encourage "counter speech" and step up the monitoring of xenophobic commentary.
The police have launched an investigation into the incident.
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