The Turkish public will vote on April 16 on whether to create a presidential system which Ankara says will be like that in France or the United States and ensure political stability.
But critics say the system will further weaken parliament and herald one-man rule by Erdogan, Turkey's strongman president.
Ahead of the vote, Turkish politicians' trips to Germany have sparked controversy, notably a rally by Prime Minister Binali Yildirim in the western city of Oberhausen to garner support for a 'Yes' vote.
"They don't want Turkey to campaign here, they are working for a 'No'," Foreign Minister Mevlut Cavusoglu told reporters in the Turkish capital today.
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"They want to get in the way of a strong Turkey."
But the German government today denied having anything to do with the municipal decision to block the rallies.
"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.
As the political fallout continued, the western German town of Gaggenau which cancelled a rally by Turkish Justice Minister Bekir Bozdag said it received a bomb threat early today.
"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 yesterday, 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.
"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.
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