Since the attempted power grab on the night of July 15th, skirmishes have broken out between backers and opponents of Erdogan in Germany, home to Turkey's biggest overseas diaspora.
Pro-Erdogan activists have stormed locations popular with followers of US-based preacher Fethullah Gülen, whom Ankara accuses of masterminding the putsch attempt, while critics of the Turkish president have also complained of abuse and threats against them on social media.
On Sunday, up to 30,000 people are expected to answer a call to take to the streets issued by a pro-Erdogan group, the Union of European-Turkish Democrats (UETD), according to police.
The North-Rhine Westphalia state, where Cologne is located, is home to about one third of Germany's three-million strong Turkish community.
Ahead of the march, security services have sought to head-off any potential clashes between pro- and anti-Erdogan groups.
Cologne's police chief Jürgen Mathies warned: “One thing I want to make clear is that we will intervene against any kind of violence quickly, decisively and forcefully.”
Some 2,000 officers will be deployed to keep the peace, including several Turkish speakers.
The tension comes at a time when relations between Germany and Turkey are already strained over the German parliament's recent decision to brand the World War I-era Armenian massacre by Ottoman forces a genocide.
German politicians led by Chancellor Angela Merkel have issued strongly worded statements against Erdogan's crackdown following the putsch – with more than 16,000 soldiers, police, judges, prosecutors and journalists detained.
The hardline response “flouts the rule of law”, Merkel's spokesman Steffen Seibert has said, also blasting “revolting scenes of caprice and revenge” in the wake of the failed coup.
At the same time, Ankara is demanding that Germany extradite suspects linked to 75-year-old Gülen, who has strongly denied any involvement in the botched coup.
Erdogan enjoys a large support base in Germany, home to some 1.5 million people with Turkish nationality who can vote in Turkish elections.
His AKP party garnered 60 percent in the country in last November's election, a bigger share of the vote than in Turkey.
Germany's integration commissioner Aydan Ozoguz underlined Erdogan's influence, saying: “I am seeing with concern that the relationships of people living here with Turkey are being massively exploited politically.”
At the same time, a substantial number of Kurds have also made Europe's biggest economy their home.
In the Berlin district of Kreuzberg, nicknamed “Little Istanbul”, Turkish flags have been flown prominently since the putsch bid.
German media have published reams of interviews with pro-Erdogan youths who were brought up in Germany and visit Turkey only during summer holidays.
Urging restraint, the state premier of the Cologne region, Hannelore Kraft, told Turkish immigrants: “Do not import a domestic political conflict to the region where you have chosen to live.”
But Gokay Sofuoglu, the chairman of the Turkish Community in Germany, said families were being torn apart by conflicting loyalties.
“Friendships will be terminated. And even within families, there are problems,” he told the national news agency DPA.
Ahead of Sunday's planned protest, he said: “I can only call for moderation”.