German police raided the homes yesterday of four Islamic clerics suspected of playing a key role in a spy network informing on opponents of the Turkish president.
German Justice Minister Heiko Maas said the four men were members of Ditib, Germany's largest association of mosques.
The raids, in the states of North Rhine-Westphalia and Rhineland-Palatinate, could further strain relations between the two NATO allies, which need each other to tackle issues ranging from militant Islamist attacks to Europe's migrant crisis.
According to a December report by Turkish state media, Diyanet has used imams in 38 countries to gather intelligence on suspected followers of Gülen, the US -based preacher the government accuses of masterminding the failed July coup.
The four religious leaders allegedly passed on information through the Turkish consulate in the western city of Cologne to the Turkish Religious Affairs Directorate, known as Diyanet, prosecutors said.
Turkish authorities on Wednesday detained 27 police officers in eight provinces across the country, the state-run Anadolu news agency said, part of the widening crackdown following a failed coup attempt in July.
"Whoever uses Islam as a cover for espionage can not rely on freedom of religion", Mr Maas, the justice minister, said.
The raids come after the federal prosecutor launched a probe last month following growing complaints of harassment in Turkish mosques.
Germany's integration commissioner, Aydan Ozoguz, told Bild daily that with such events, Ankara was "deepening divisions" among Turks living in Germany.
Last September, the Turkish religious authority requested information from its representatives overseas on FETO activities in their countries, in order to prepare a comprehensive report to be discussed at the 9th Eurasia Islamic Council meeting.
Earlier this week, the Austrian Interior Ministry announced an investigation into concerns raised by Greens Parliamentarian Peter Pilz that Turkish diplomatic offices around the world were gathering information to try to undermine organizations loyal to Gulen.
Gulen, who has lived in self-imposed exile in Pennsylvania since 1999, has denied the charge and condemned the coup.
The government says the measures it took are necessary to clean the state of the "virus" of Gulen's movement, which encourages its members to work in public services.
Chancellor Angela Merkel's government has repeatedly criticised the scale of the crackdown and urged Mr Erdogan to safeguard civil liberties.