Germany summoned the Turkish ambassador on Wednesday to protest against Ankara's arrest of six activists, including a German national.
Chancellor Angela Merkel has also condemned the detentions.
A Turkish court jailed Amnesty International's Turkey director and five others, awaiting trial for alleged aid they gave to an armed terror group.
Merkel called for the release of the German citizen, identified as Peter Steudtner.
The six people were detained in a July 5 police raid on a hotel on an island off Istanbul, where they were attending a digital security workshop.