"No tolerance for Salafists" said conservative daily Die Welt after a small group of men, wearing orange vests with "Sharia Police" written on them, went on a series of "patrols" in the western city of Wuppertal.
Seeking to enforce their austere moral code, they told nightclub-goers to refrain from drinking alcohol and listening to music and arcade customers not to play games for money.
A video circulating online shows among them Sven Lau, a German Salafist convert who claims to be one of those behind the patrol idea.
Police said yesterday the patrols took place last week and urged people to contact them if they saw suspicious behaviour linked to Salafist activity.
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No arrests have been made so far, but political leaders warned they would crack down on the Islamist patrol if it took its campaign any further.
"We will not tolerate an illegal parallel justice," warned Justice Minister Heiko Maas.
"Sharia law is not tolerated on German soil," Interior Minister Thomas de Maiziere told Saturday's Bild newspaper.
Bavaria state's interior minister Joachim Herrmann described it as a "direct attack by the Salafists on our rule of law" in comments published in Monday's Bild daily.
Volker Kauder, the parliamentary group leader of Merkel's conservatives, argued that the police alone was responsible for upholding public order.
"Therefore we must examine a ban of these supposed guardians of Islamic virtues," he told the Welt am Sonntag.
The head of the Central Council of Muslims in Germany has also condemned the action by the Salafists in Wuppertal.
German intelligence last year voiced concern over the growing number of Salafists, who espouse an austere form of Sunni Islam, and said they numbered around 4,500 in the country.