The Turkish authorities have arrested a German-Turkish journalist they accuse of producing terrorist propaganda and undermining the government.
Deniz Yucel, who works for Die Welt, was detained almost two weeks ago and has since been held in police custody.
He had earlier reported on leaked emails that allegedly highlight the influence wielded by President Recep Tayyip Erdogan's son-in-law.
German Chancellor Angela Merkel called the move "disappointing".
"We will continue to insist on a fair and legal treatment of Deniz Yucel and hope that he will soon regain his freedom," she said.
Foreign Minister Sigmar Gabriel said Mr Yucel's arrest showed in "glaring light" the differences between Germany and Turkey on matters of freedom of the press and freedom of opinion.
Die Welt says Mr Yucel, 43, is the first journalist with German nationality to be held in pre-trial detention since Mr Erdogan's AKP party took power in 2002.
Several other journalists have also been detained in connection with the emails, purportedly obtained by left-wing hackers from the private account of Berat Albayrak, who is the country's energy minister.
Pre-trial detention in Turkey can last up to five years, Die Welt says.
Many Turkish writers and journalists have been arrested in a widespread crackdown that followed a failed July coup against Mr Erdogan.
More than 100,000 people have also been sacked or suspended from Turkey's police, military, civil service and private sector since the failed coup and tens of thousands arrested.
The Turkish government says the measures are necessary given the security threats it faces but critics say Mr Erdogan is using the purges to stifle dissent.