Cologne - Tens of thousands of supporters of Turkish President Recep Tayyip Erdogan are planning to rally in the German city of Cologne on Sunday, as tensions over Turkey's failed coup put authorities on edge.Since the attempted military power grab on the night of July 15, skirmishes have broken out between backers and opponents of Erdogan in Germany, home to Turkey's biggest diaspora.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.At the same time, several smaller counter-demonstrations, including one by the far-right activists, are due to take place, sparking fears of clashes.Ahead of the march, Erdogan told the West to "mind your own business" over criticism of the post-coup crackdown. He also took aim in particular at Germany and Austria, accusing them of stifling freedom of expression."Our citizens want to hold meetings, go on marches but they stop them. They have even gone so far as to stop Turkish flags being flown from houses. These are the kind of democracies they are," he charged on Friday.Ahead of Sunday's demonstrations, security services in Germany sought to head-off any potential violence between pro- and anti-Erdogan groups.Cologne's police chief Juergen Mathies warned: "One thing I want to make clear is that we will intervene against any kind of violence quickly, decisively and forcefully."Some 2 700 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.'Don't import Turkey's problems'German politicians led by Chancellor Angela Merkel have issued strongly worded statements against Erdogan's crackdown following the putsch -- with more than 18 000 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 US-based preacher Fethullah Gulen, whom Erdogan accuses of masterminding the putsch. The 75-year-old has strongly denied any involvement.The strife has spilled over into Germany, as pro-Erdogan activists have stormed locations popular with Gulen's followers.Critics of the Turkish president have also complained of abuse and threats against them on social media.German Foreign Minister Frank-Walter Steinmeier on Saturday warned in an interview with Sueddeutsche Zeitung daily: "It is not right to bring Turkey's domestic political tensions here... and intimidate people who have other political convictions."Erdogan enjoys a large support base among the diaspora in Germany, home to some 1.5 million people with Turkish nationality who can vote in Turkish elections.His AKP party garnered 60% 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.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".