German police said they were checking the identities of hundreds of Muslims leaving Friday prayers in the city of Bochum in a search for potentially violent Islamists.
A police spokesman in the western city said about 400 people were being checked outside two mosques as they left after prayers.
"These are not raids on mosques. These are identity checks outside the mosques," the spokesman said.
He said the worshippers were co-operating with police and so far no arrests had been made.
"The police have no indications of any concrete plans for an attack in Bochum. But there are serious indications that there are people at these two mosques who are under suspicion of belonging to a circle of militant Islamic extremists," police said in a statement.
Germany has a Muslim population of more than three million, and security services closely track the activities of what they describe as a small minority of potentially violent Islamists.
Germany was shaken in 2001 by revelations that three of the suicide pilots who led the September 11 attacks on the United States were Arab militants who had lived for years as students in the port city of Hamburg.