The German government has warned its citizens and firms they face the risk of "arbitrary" arrest in Turkey.

"People who are travelling to Turkey for private or business reasons are urged to exercise increased caution," the German foreign ministry said.

Firms face investment risks in Turkey because of Turkish legal deficiencies, the ministry said.

Germany protested to Turkey's Berlin ambassador on Wednesday over the arrest of six human rights activists.

The six include a German citizen, Peter Steudtner, and Amnesty International's Turkey director, Idil Eser.

The Turkish foreign ministry hit back, calling Germany's complaint "unacceptable" and "direct interference in the Turkish judiciary"

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