Turkey's MIT intelligence service has been spying on hundreds of people in Germany suspected of being part of an anti-Erdogan movement, reports say.
The scale of espionage emerged after the head of MIT handed a list to German foreign intelligence chief Bruno Kahl, according to German media.
As well as addresses and phone numbers, the Turkish official apparently also provided surveillance photos.
The two countries are already involved in a row over a Turkish referendum.
German authorities, in common with several other EU states, barred Turkish ministers from holding rallies to campaign for increasing the powers of President Recep Tayyip Erdogan.
An estimated 1.4m Turks are eligible to take part in the 16 April referendum, and voting in Germany and five other European countries began on Monday.
Since Turkey's authorities put down a coup in July 2016, some 41,000 people have been arrested on suspicion of following the movement of US-based cleric Fethullah Gulen. Turkey has accused him of organising the coup.
According to Sueddeutsche Zeitung and several public broadcasters, the head of MIT, Hakan Fidan, handed Bruno Kahl a list of 300 individuals and 200 organisations at a security conference in Munich in February.
The apparent aim was to persuade Germany's authorities to help their Turkish counterparts. But the result was that the individuals were warned not to travel to Turkey or visit Turkish diplomatic addresses within Germany.
Police in the western state of North Rhine-Westphalia told German TV that the lists had to be taken seriously, and there was an angry response from senior German figures.
"Outside Turkey I don't think anyone believes that the Gulen movement was behind the attempted putsch," said German spy chief Hans-Georg Maassen.
"At any rate I don't know anyone outside Turkey who has been convinced by the Turkish government."
And Lower Saxony Interior Minister Boris Pistorius went further, suggesting that "we have to say very clearly that it involves a fear of conspiracy you can class as paranoid".
Last week, Swiss prosecutors said they were investigating allegations that Turks critical of the Erdogan government were spied on at a lecture in Zurich.