Turkey and Germany have been locked in a deepening diplomatic row after some cities banned Turkish ministers from speaking to rallies of expatriate Turks on public safety grounds.
Merkel stressed that such insults must stop - "no ifs, no buts" - and that Germany reserved the right to "take all necessary measures, including reviewing the permissions" for campaign events it had already granted.
Ankara has said such behaviour was reminiscent of Nazi Germany and also raised alarm over what it sees as rising racism and Islamophobia.
Erdogan pointed to the European Union criticism of Turkey's state of emergency following last July's attempted coup and told the bloc to "mind its own business".
The Turkish foreign ministry on Tuesday said it had called in the German envoy to protest comments by Berlin's foreign intelligence chief expressing scepticism that US-based Islamic preacher Fethullah Gulen was behind the July 15 failed coup..
While some commentators believe Turkish criticism of Europe is created to rally nationalist support for a "yes" vote and could subside after the referendum, the president's warning of possible threats to Europeans was likely to further test ties with Europe, Turkey's No. 1 trading partner. "Well, I am with my people", Erdogan said.
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Germany's Frank-Walter Steinmeier used his first speech as president on Wednesday to warn Turkey's head of state Tayyip Erdogan that he risked destroying everything his country had achieved in recent years.
"President Erdogan, you are jeopardizing everything that you, with others, have built", he said, adding that he would welcome "credible signs" to ease the situation.
"Measures must be taken so that they live in friendship with foreigners".
The official, who spoke on customary condition of anonymity, said the decision has nothing to do with the animosity between European nations and Turkey.
A man reads an issue of Gunes, a Turkish pro-government daily newspaper, with on its front page German Chancellor Angela Merkel depicted in Nazi uniform with a Hitler-style mustache, labeling the German leader "She Hitler", on March 17, 2017 in Istanbul, Turkey.
Erdogan, who accused Yucel of being a "terror agent" and taking refuge with German consular officials for a month, said Merkel had asked for him to be allowed to leave Turkey during her February visit.