"German Constitutional Court's decision on the anti-coup rally in Cologne is an utter backsliding in freedom of speech and democracy," Omer Celik wrote in English on his official Twitter account.

The rally has been called in support of Turkey's democracy following a failed military coup on July 15-16. German media reports said authorities had intervened to prevent Erdogan from addressing the rally via video conference due to concerns over public order.

Germany is home to Europe's largest ethnic Turkish diaspora.