Police said they stepped up their presence in the city and, as a precaution, increased security for the Jewish community.
They said Saturday evening, several hours after issuing the warning, that they had carried out searches on an unidentified suspect and at an Islamic culture center. One person was arrested, they added.
The warning comes amid heightened concern in Europe about terrorism following attacks this year in Paris and Copenhagen.
In mid-February, police in the northern German city of Braunschweig canceled a Carnival street parade over fears of a terrorist attack.
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