But one such venerable institution, the 112-year-old Cafe Prückel, is in hot water over an incident that threatens to tarnish the Austrian capital's reputation for tolerance just as it seeks to attract more gay tourists.

And it all comes less than a year after the country's bearded transvestite Conchita Wurst — with her long glossy mane, heavy makeup and stiletto heels — helped put Vienna on the gay map by winning the Eurovision Song Contest.

In the now infamous episode on January 6, lesbian couple Eva Prewein and Anastasia Lopez, 26 and 19, engaged in what the women call merely a "Begrüssungskuss" — a "kiss of greeting" — at the cafe.

The waiter was less than impressed and the couple say they were refused service for more than two hours and then barred when they kicked up a fuss.

Lopez told Austrian newspapers that Christl Sedlar, manager of the cafe on Vienna's famous Ringstrasse boulevard, informed the outraged pair that "diversity such as this belongs in a brothel, not in a traditional coffee house".

"We don't need any canoodling. Why was it absolutely necessary for the couple to show that they were an item," Sedlar told the Kurier daily.

All might have ended there but Sedlar's parting shot, unwisely as it now turns out, was that if the pair didn't like how they had been treated then "you girls could always go on Facebook".

And so they did. With the help of several LGBT (lesbian, gay, bisexual, and transgender) groups — and a Facebook page, they organised a demonstration outside the Prueckel for this Friday evening.

By Thursday — the day before — 7,300 people had announced their attention to take part. Co-organiser Tamara Mittermann, head of the Axis of Critical Students (AKS), said she expects around a third or a half of these to actually turn up.

"This is not the first homophobic incident in a Vienna cafe," Mittermann told AFP. "Austrians feel like they are very tolerant … but in fact this isn't the case. There is for example no legal protection against discrimination in the private sphere."

Manager Sedlar issued an apology of sorts on Thursday — but it looks like this will do little to calm tempers.

Pink euros

Vienna's cafe culture dates back to the end of the 17th century — legend has it that the retreating Turkish army left coffee beans behind in 1683 — and is included in UNESCO's list of intangible cultural assets.

Their heyday was in the late 19th and early 20th century when they were a haven for Europe's cultural and intellectual elite like Sigmund Freud and Gustav Mahler — as well as the likes of Lenin, Trotsky, Tito and a certain Adolf Hitler.

Although this belle epoque is long gone and the number of cafes have dwindled, they remain popular hangouts for locals and tourists alike, with visitors allowed to spend hours poring over the newspapers and ordering very little.

But Vienna's tourist board is also pulling out all the stops, like other European destinations, to attract gay tourists. Last year 80,000 people voted Vienna as their favourite cultural destination on website GayCities.com.

Although it may be a little less racy than other cities, there is also "fun to be had after dark, with plenty of fetish parties, saunas and gay clubs to keep even the most energetic boy or girl happy," the website said.

"We have been  targeting LGBT tourists since 1998 and this unwelcome episode in Cafe Prückel is definitely counter-productive," Vera Schweder from the Vienna tourist board told AFP.

"But I think this is an exception, generally this is a very welcoming place."