The group, which wore orange vests with "Shariah Police" on the back, visited local nightspots Wednesday to persuade young people to abstain from drinking, gambling and other offenses it deemed not in accordance with Islam.
They were arrested on charges of unlawful assembly and use of a uniform in public, a police spokeswoman said. It was not clear if the vigilantes were specifically targeting people they regarded as Muslim.
"Can a vest and a name really cause so much headache? What have all the grumblers now coming out of the woodwork done for wayward youth? Do you realize how many are buying and consuming drugs? If you are so honest and good, where is your shrieking and outrage?" the a post on the Shariah Police Facebook page said Wednesday.
Police in Wuppertal, a city of 350,000 in western Germany, pledged in a statement to crack down on anyone engaged in intimidation or imitating police. The statement noted attempts by members of the conservative Salafi Islamist sect to influence and recruit young people.
"Any conduct that intimidates, provokes or makes people insecure will not be tolerated. These 'Sharia Police' have no legitimacy," Police Chief Birgitta Radermacher said.
"In this way they are demonstrating that they do not recognized our law-based state," a spokesman of Germany's interior ministry commented to the Rheinischer post newspaper.