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BERLIN—A diplomatic rift sparked by Germany's handling of a Turkish demonstration deepened on Monday, the latest sign of worsening relations between Turkey and the European Union since the failed Turkish coup in July.

The government in Ankara summoned a senior German diplomat Monday afternoon to explain why the country's top court had barred Turkish President Recep Tayyip Erdogan from addressing by video a crowd of around 40,000 of his supporters in the German city of Cologne the day before.

Citing the risk of unrest at the rally, German police had decided not to let Mr. Erdogan appear via a live link. German courts upheld the decision over the weekend after protest organizers challenged it, prompting Turkey's EU Affairs Minister Omer Celik to describe the ruling as "utter backsliding in freedom of speech and democracy."

The growing tensions between two major players in Europe's migrant crisis reflect the high stakes for the EU as it struggles to strike a balance between condemning Mr. Erdogan's postcoup crackdown and maintaining good relations with a critical partner in grappling with terrorism and migration.

Turkish and German politicians have since traded accusations of bias and blackmail via television and social media, displaying the heightened mistrust between the countries over what Mr. Erdogan's European critics see as autocratic repression in Turkey—and his Turkish supporters see as unfair criticism of their country.

"This is the lowest point I've ever seen in German-Turkish relations," said Burak Copur, a political scientist and scholar of German-Turkish relations at the University of Duisburg-Essen. "And therefore we are also heading towards a clash between the EU and Turkey."

The stakes are particularly high for German Chancellor Angela Merkel, who has sought to avoid riling Mr. Erdogan, her ally in Berlin's efforts to curb the recent inflow of migrants.

After two terror attacks committed by asylum seekers in the past two weeks, Ms. Merkel faces new pressure at home to contain the flow of migrants to Germany. Her open-door migrant policy allowed over one million migrants to come to here last year and a March deal reached with Turkey to take back migrants who cross into Europe illegally from Turkish shores has contributed to a significant drop in the number of migrants crossing into Europe.

Politicians in Turkey have threatened to cancel the pact unless Europe delivers promised concessions soon, especially visa-free travel in Europe for Turkish citizens. But EU officials say Turkey must first meet criteria that include addressing the treatment of political opponents, which the crackdown is placing in question.

"Under no circumstance should Europe let itself be blackmailed," Germany's Deputy Chancellor Sigmar Gabriel told reporters on Monday, after Turkey's Foreign Minister Mevlut Cavusoglu told a German newspaper over the weekend that visa-free travel must be granted by October if cooperation on migration is to continue.

Turkey's Mr. Celik criticized the German judicial ban on Mr. Erdogan's video address in a series of Twitter posts, turning around accusations of censorship and curbs on other civil liberties that German politicians have leveled at Mr. Erdogan. It is "such a shame to see that [the] EU fails in upholding democracy and showing solidarity with a candidate country in the face of a coup threat," he wrote.

A senior Turkish official said Turkey was still committed to working with Europe on migration and other issues but expected its European partners to hold up their end of the deal.

In Brussels, EU officials are concerned that the events in Turkey could harm relations so much that last year's migration crisis could reignite. More than a million refugees and other migrants arrived in Europe in 2015, mostly via the Middle East, sparking a political crisis in the EU that has been contained recently mostly with Turkey's help.

The average number of migrants crossing the Aegean Sea from Turkey to Greece has dropped to 89 a day since the EU-Turkey migration deal was signed in March, compared with daily average arrivals of about 1,740 in the months before the agreement, according to the European Commission.

Commission President Jean-Claude Juncker warned over the weekend that the migration deal could fall apart. He expressed concern about "torture and arbitrary detention" of alleged coup plotters and political dissidents, which he said were just as big a problem for EU-Turkey relations as some Turkish politicians' talk of reintroducing the death penalty for coup participants.

In France, the latest political turmoil has only reinforced doubts about whether Turkey should get visa-free travel, a French official said.

In Germany, home to the largest Turkish diaspora, thousands of Turks and Germans of Turkish descent have marched in support of Mr. Erdogan since the coup attempt.

Demonstrations have been mostly peaceful, but a series of violent incidents in Germany against critics of Mr. Erdogan have raised German officials' fears that Turkey's political troubles could spill over into their country.

The two countries have had a series of diplomatic rows in recent months. In April, the German chancellor had to weigh relations with Ankara and press freedoms in Germany when Turkey pursued the prosecution of a prominent German comedian for reciting a crude poem mocking the president. Ms. Merkel was heavily criticized at home for allowing a criminal investigation into the comedian's performance to move forward. In June, Germany's parliament approved a motion calling the killing of more than a million Armenians by Ottoman Turks a genocide, angering Turkey yet again.

"Turkey has been an important and good partner for Germany and Europe for centuries," Martin Schä fer, a spokesman for the German Foreign Ministry, said Monday. "We have had bumpy phases now and again but also phases that were extraordinarily good. Currently, we are in a somewhat bumpy phase."

Valentina Pop in Brussels and Emre Peker in Istanbul

Write to Ruth Bender at Ruth.Bender@wsj.com

(END) Dow Jones Newswires

August 01, 2016 19:15 ET (23:15 GMT)

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