Turkish President Recep Tayyip Erdogan likened Germany's cancellation of public appearances by two of his ministers to Nazi practices, aggravating frictions that had already flared over Turkey's human rights record.
Germany's decision has "nothing to do with democracy," Erdogan said at an event in Istanbul, state news agency Anadolu reported.
"Recent practices are no different from the Nazi ones of the past."
The German Government press office didn't immediately respond to calls seeking comment, but Merkel's Christian Democratic Union deputy parliamentary leader Michael Fuchs called Erdogan's Nazi references "outrageous".
"The Nazi comparison, the slur on German democracy is as far removed from reality as it gets but we shouldn't be provoked," Fuchs said, adding that the German Government will move on and ignore the comments to de-escalate tensions.
Ties between Turkey and Germany, as well as the rest of the European Union, have been strained over Erdogan's sweeping crackdown on opponents following the failed attempt to topple him in July.
Ankara has jailed a German-Turkish reporter whom Erdogan described as a spy, and is pressing Germany to extradite fugitive Turkish military officers involved in the coup attempt.
New tensions emerged last week after the municipality of Gaggenau in southwest Germany revoked its permission for Turkish Justice Minister Bekir Bozdag to campaign for a planned referendum to expand Erdogan's powers, citing concerns of overcrowding. Bozdag cancelled his trip and a scheduled meeting with his German counterpart.
Economy Minister Nihat Zeybekci's planned address today to a Turkish community in the western German city of Cologne was also cancelled by local authorities citing security concerns. Zeybekci travelled to Germany to attend another event.
Turkey's conduct in the aftermath of the coup has further clouded its prospects for joining the EU.
Austrian Chancellor Christian Kern called for an end to Turkey's EU membership bid in a Bild am Sonntag interview published today. Kern said Turkey has distanced itself from democratic standards and the principles of rule of law, and called on the EU to rethink billions of euros in aid intended to promote Turkey's membership campaign.
The European Union is a critical trade partner for Turkey, taking 47 per cent of its total exports in January. Germany was the No. 1 export destination for Turkish goods, accounting for 10 per cent of exports, according to Turkey's official statistics agency.