President Recep Tayyip Erdogan on Friday accused Berlin of "aiding and harbouring" terror after German authorities banned rallies courting support from Turkish expatriates for a constitutional change to expand Erdogan's powers.
Ankara and Berlin were locked in acrimony and Erdogan also claimed a detained correspondent working for Germany's Die Welt newspaper was a spy, an accusation Germany said "makes no sense."
"They need to be put on trial for aiding and harbouring terror," Erdogan said, criticising German authorities who had allowed outlawed Kurdish leaders to speak but had blocked rallies where Turkish ministers sought to address the Turkish community.
The president also claimed that Die Welt correspondent Deniz Yucel, who was detained on Monday on terror-related charges, was a "German agent."
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 drag Turkey into one-man rule.
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.
An incensed Erdogan, who was prevented from adressing a rally in Cologne by video link after a failed coup in July last year, lashed out at Germany.
"They allow Cemil Bayik to speak from (the) Kandil" mountains in Iraq, he said, referring to the outlawed Kurdistan Workers' Party (PKK) leader.
Earlier, Foreign Minister Mevlut Cavusoglu also accused German officials of failing to "honour democracy" and of allowing "terrorists" to speak but denying the same right to Erdogan.
"They don't want Turkey to campaign here, they are working for a 'No'," he said. "They want to get in the way of a strong Turkey."
German Chancellor Angela Merkel rejected Turkey's accusations, saying the decisions were "taken by municipalities, and as a matter of principle, we apply freedom of expression in Germany".
Cavusoglu and German Foreign Minister Sigmar Gabriel, speaking by phone, agreed to meet next Wednesday, a senior Turkish official said.
- Another rally cancelled -
As the political fallout continued, 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 cancelled 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.
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.
- 'German agent' -
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.
The Die Welt correspondent was charged on Monday by an Istanbul court with spreading terrorist propaganda and inciting hatred.
"That person had hidden at the German consulate for a month as a PKK representative, as a German agent," Erdogan said, adding that Germany had initially refused to hand him over for trial.
Speaking earlier in the day, Merkel said Berlin was right to criticise Ankara over press freedom.
"I also think that it was right of us to criticise any restrictions on press freedom."
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.