"It is notable with what intensity and ruthlessness the people living abroad are being investigated," said Boris Pistorius, interior minister of the northern German state of Lower Saxony.
"It's intolerable and unacceptable," he said at a press conference.
Ankara had asked Berlin to help spy on about 300 alleged Gulen supporters, Pistorius said, adding that the list was handed to Germany's spy service, which turned it over to state governments.
Turkish authorities were acting with "something close to paranoia," he said, adding that "all Gulen supporters are assumed to be terrorists and enemies of the state even though there is not the tiniest scrap of evidence."
"Until today, we have no evidence whatsoever that Gulen supporters have violated any rules in any way."
Although US-based cleric Gulen, 75, has always denied charges that he was involved in the failed coup last July to overthrow Erdogan, Ankara has cracked down hard on his followers.
In February, German police raided the homes of four Turkish Muslim preachers suspected of spying on alleged Gulen supporters for Erdogan's government.
Erdogan has in turn accused Germany of harbouring Kurdish and other "terrorists", claiming that Berlin is refusing to hand over alleged suspects.
Germany's foreign intelligence chief Bruno Kahl also raised heckles in Turkey last week when he said he was unconvinced Gulen was behind the failed coup of July 15.
Kahl said that the putsch was launched by a "part of the military" that expected to be targeted in an ongoing government purge.
The latest German accusations came as ties are already badly strained over a wide range of issues surrounding human rights, particularly after the failed coup.
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