"We see that Turkey under Erdogan is on the way to a one-man state," Schulz told the Cologne daily, Kolner Stadt-Anzeiger, citing last week's lifting of parliamentary immunity for opposition MPs. He also took aim at changes in the prime ministership, after Ahmet Davutoglu stepped aside and was replaced by Erdogan ally Binali Yıldırım, POLITICO reports.

Schulz said there can be no visa liberalization for Turkish citizens traveling in Europe until Ankara meets all the EU's conditions, including changes to anti-terror legislation. Erdogan has previously said he will not move on that point.

"We have to make it clear that we do not idly accept the monopolization of power in one man's hands," Schulz said.

Schulz also spoke out against scrapping the EU-Turkey refugee deal.

Read alsoTurkish parliament strips MPs of immunity in blow to Kurdish opposition"The refugees would have to pay the price in the end," he said. "We need a good agreement. We will therefore continue to cooperate with Turkey, but we must not remain silent."

The statements come ahead of German Chancellor Angela Merkel's visit to Istanbul Monday to attend a summit on refugees and attempt to save the EU's refugee arrangement with Ankara.

Read alsoEU states agree to make it easier to suspend visa-free travel"I am convinced that it's in German, European and Turkish interests, and especially in the interests of those who are fleeing war and persecution," Merkel said of the arrangement in an interview published Sunday, with Frankfurter Allgemeine Sonntagszeitung.