Berlin summoned Wednesday Turkey's ambassador over the detention of German national Peter Steudtner, who was remanded in custody this week as part of an ongoing anti-terror probe.
"The Turkish government needs to immediately and directly hear the German government's outrage and incomprehension as well as its crystal-clear expectations in the case of Peter Steudtner, and this time without diplomatic niceties," ministry spokesman Martin Schaefer said.
German Foreign Minister Sigmar Gabriel would interrupt his summer holiday in order to respond to what the spokesman referred to as a "dramatic escalation."
The ten terror suspects, including German Steudtner and a Swedish national, were detained on July 5, while they were at a meeting on Büyükada, an island near Istanbul.
Charges against the suspects have not been formally announced yet but Turkish media outlets reported that one of the Turkish suspects was linked to the Gülenist Terror Group (FETÖ), which was behind the July 15 coup attempt in 2016.
They were reportedly planning provocative events that would fuel unrest across Turkey similar to the notorious Gezi Park riots in 2013.
According to a report by Akşam daily, the suspects gathered at a hotel in Büyükada to organize riots as the main opposition Republican People's Party (CHP) Chairman Kemal Kılıçdaroğlu's protest march neared Istanbul earlier this month.