The failed coup against President Recep Tayyip Erdogan has stoked tensions in Germany's three-million strong Turkish community, prompting demonstrations and threats that have put the authorities on edge.

After days of purges amid a crackdown on suspects and Erdogan's rivals in Turkey, German Interior Minister Thomas de Maiziere issued a stern warning to partisans among the world's largest Turkish diaspora.

"We don't want these kinds of conflicts to play out in Germany with violence in the streets," he told ZDF public television.

However supporters and opponents of Erdogan in Germany have vented their anger in the wake of a turbulent week in Turkey which saw an attempted coup crushed and Ankara detaining or sacking tens of thousands of people.

Turkish authorities imposed a state of emergency, strengthening state powers to round up suspects and suspending a key European rights convention, prompting serious concerns in the EU about the rule of law.

Those worries have been compounded in Germany by security fears, as tensions between backers of Erdogan's ruling Justice and Development Party (AKP) and his Islamic preacher Fethullah Gulen, whom Ankara blames for the coup, threaten to boil over.

- 'Down with traitors' -

The BDK federation representing 15,000 police officers cautioned there was a "very, very large" potential for the situation to explode.

"All Erdogan has to do is snap his fingers to get people to confront each other in the streets in a very emotional way," its vice president Sebastian Fiedler told ZDF.

In the days after the uprising, Turks in Germany received appeals on social media to denounce suspected Gulen supporters, including a hotline number to call at the Turkish president's offices, the Frankfurter Allgemeine Zeitung reported.

In the western city of Gelsenkirchen on Wednesday, 150 people loyal to Erdogan stormed a cafe popular with Gulen backers, breaking a window and scuffling with patrons.

And mosques with links to the Turkish government hung placards reading "Down with traitors to the fatherland".

During the night of the attempted coup, about 3,000 people massed in front of the Turkish embassy in Berlin, waving Turkish flags and holding aloft pictures of Erdogan.

Local media reports said members of a Turkish far-right organisation, the "Grey Wolves", and other nationalist groups joined the rally as more than 200 police officers kept the peace.

A second pro-Erdogan demonstration in the German capital went off without incident Saturday while a third, expected to draw 15,000 people, will be held in the western city of Cologne at the end of the month, police said.

Nearly all of Germany's Turks are descendants of a "guest worker" programme launched in the 1960s to bring manpower to then West Germany.

- 'Liar, traitor' -

Meanwhile, however, hatred between the two sides has exploded on social media.

"All those who criticise Erdogan are immediately considered supporters of Gulen or the PKK (the outlawed Kurdistan Workers' Party)," said a leader of the opposition Greens party, Cem Ozdemir, himself the son of a Turkish guest worker.

"Erdogan may be able to do what he wants in Turkey, and that's bad enough, but he can't do it in Germany."

A member of the Berlin state legislature, Erol Ozkaraca, a vocal Erdogan critic, said he had been subject to taunts and slurs.

"Liar, traitor, Gulen supporter -- it's all a lot of rubbish," the Social Democrat told AFP.

Ercan Karakoyun, the head of the Foundation for Dialogue and Education, an organisation with close links to Gulen, has faced even more fervent abuse.

"It is threatening to see how there has been a campaign against the Gulen movement in Germany too conducted with rumours and conspiracy theories," he told the Stuttgarter Zeitung.

Erdogan enjoys strong support among the 1.55 million Turkish citizens living in Germany. During the Turkish election last November, his AKP garnered around 60 percent of the vote, an even stronger share than in Turkey.

And although many were born in Germany or lived here for decades, many ethnic Turks say they feel more engaged with political life in Turkey.

"With his propaganda and media presence, (Erdogan) creates the impression that he has made Turkey strong," Ozkaraca said.

"They (Turkish immigrants) don't have a sense of belonging to German society. We have not managed to integrate here, which leads people to identify with their country of origin."

Erdogan has recognised the importance of his base in Germany and even held stadium-sized campaign rallies in recent years in cities such as Berlin and Duesseldorf.