Ankara and Berlin were locked in acrimony Friday after several German towns scrapped rallies courting support from Turkish expatriates for a constitutional change that will expand President Recep Tayyip Erdogan's powers.
German Chancellor Angela Merkel rejected Turkey's bitter accusations that her government had had a hand in scrapping the rallies.
Turks vote on April 16 on whether to create a presidential system -- a change that the government says will ensure political stability, but which critics say will herald one-man rule by Erdogan.
In the runup to the referendum, controversy has flared over politicians' trips to Germany, where they have been seeking "Yes" votes from the millions of people of Turkish descent.
One such event was a rally by Prime Minister Binali Yildirim in the western city of Oberhausen.
On Thursday, local authorities blocked rallies by two more Turkish ministers, prompting a furious response from Ankara which promptly summoned the German envoy to protest.
"They don't want Turkey to campaign here, they are working for a 'No'," Foreign Minister Mevlut Cavusoglu told reporters in the Turkish capital on Friday.
"They want to get in the way of a strong Turkey."
But the German government denied having had anything to do with the cancellations.
- Bomb threat -
"That is a decision the federal government has absolutely no influence on... because it falls under local or state jurisdiction on which we have zero influence," said foreign ministry spokesman Martin Schaefer.
Merkel said the decisions were "taken by municipalities, and as a matter of principle, we apply freedom of expression in Germany".
Austria has also said it would not allow any campaign-related events.
Cavusoglu and German Foreign Minister Sigmar Gabriel, speaking by phone, agreed to meet next Wednesday, a senior Turkish official said.
As the political fallout continued on Friday, a third German town -- Frechen, on the outskirts of Cologne, scrapped a rally that had been scheduled for Sunday, as the venue "excludes political events," police said.
And the western town of Gaggenau, which had scrapped a rally scheduled for Thursday by Turkish Justice Minister Bekir Bozdag, said it received a bomb threat early Friday.
"The caller cited the cancellation of the event with the Turkish justice minister as a reason," local official Dieter Spannagel told AFP.
Bozdag had been due to meet the Turkish community there on Thursday, but cancelled his address after the Gaggenau authorities withdrew their consent for the Union of European Turkish Democrats (UETD) to use the hall, citing capacity problems.
Cologne city authorities also withdrew permission for the UETD to use a hall on Sunday for a speech by Economy Minister Nihat Zeybekci. But Zeybekci said he would still go ahead with the visit.
"Even if they don't allow (it), I will go from house to house to meet with our citizens in Germany," he was quoted as saying by the state-run Anadolu news agency.
Cavusoglu accused German officials of failing to "honour democracy" and of allowing "terrorists" from the outlawed Kurdistan Workers' Party to speak but denying the same right to Erdogan.
- 'Learn how to behave' -
In response, a spokeswoman for Merkel said permits for rallies were only issued by local authorities and that freedom of opinion should be valued by both countries.
"The referendum is a domestic issue for Turkey," Ulrike Demmer said, adding: "Freedom of opinion... should be respected" both in Turkey and in Germany.
Tensions have erupted previously over German criticism of the crackdown in Turkey following last year's failed coup, which has seen more than 100,000 people arrested, dismissed or sacked for alleged links to the plotters or to Kurdish militants.
Tempers on Monday flared again after 43-year-old Deniz Yucel, a correspondent for Germany's Die Welt daily, was charged by an Istanbul court with spreading terrorist propaganda and inciting hatred.
The dual national has been held in prison since February 18, with Gabriel saying the case would make "everything harder" for Turkish-German relations.
Germany is home to the biggest population of Turks outside Turkey with around three million in the country of Turkish origin, the legacy of a massive "guest worker" programme in the 1960s-70s.