In case their political motivations were in any doubt, they also tacked up a sign that read "Islamization. No thank you!" and signed it with the symbol of the Identity Movement.
Vienna police were less than amused to wake up to the sight of a national monument draped in such a provocative political protest -- but no one has been arrested for the stunt.
Martin Sellner, however, was pleased: "We try to teach patriots in Europe methods of non-violent action. That's our main principle. No violence. No real hate speech."
A clean-cut 27-year-old graphic designer, wearing black-framed glasses and brightly-colored shirts advertising Identity Movement slogans such as "Europa Nostra" -- "Our Europe" -- Sellner has been called the "hipster" of the far-right.
"We see ourselves as patriots, not neo-Nazis," he says. "We don't hate immigrants. But we also don't want to see the country change and end up minorities in our countries. We wanted to express this opinion without anti-Semitism, without the racism of the old right."
It is a carefully cultivated image: a sunny veneer masking a political ideology centered on a white Christian identity which rejects immigration and globalization.
Influx of refugees
"My generation was never asked if we want this mass immigration, this Islamization and population replacement in this country," he explains over several cups of Viennese coffee. "We were born into a society that believes we are racist for simply saying that we are becoming a minority in our own country."
In a series of posts on YouTube, Sellner suggests white Christians are on the verge of "extinction," increasingly marginalized by growing numbers of migrants in what he calls the "Great Replacement."
"My biggest fear is that at some point demographics could kill democracy," he says, "that our society by mass immigration becomes such an ethnically fragmented society that a real democracy is not possible anymore because there is no common ground of values, history and identity."
But that fear isn't supported by the numbers.
While an estimated 15% of Austria's population are foreign citizens, approximately half of those come from within the EU. For Sellner, intra-European migration -- white and Christian -- is acceptable. Non-European residents in Austria are outnumbered 10 to one.
Still, Sellner is not alone: Worries about migration reached fever pitch last year when more than a million asylum seekers crossed into Europe, at least 700,000 of them via Austria.
Many were from Syria and Afghanistan, and while most were simply passing through on their way to Germany, Sweden and elsewhere, an estimated 90,000 opted to stay and claim asylum in Austria.
Surge in far-right support
The Austrian government responded by erecting border fences, decreeing that only 3,200 migrants would be allowed to enter the country each day, and capping resettlement claims to 80 a day until an annual limit of 37,500 was reached.
Across Europe, the tide of new arrivals has prompted waves of anti-immigrant sentiment; far-right parties such as France's National Front and Austria's Freedom Party are seeing a surge in support.
Sellner is a big fan of US President-elect Donald Trump, whom he calls "God Emperor." He hopes Trump's election victory -- on a platform that included plans to build a wall along the Mexican border and a ban on Muslim immigrants -- will further inspire the far-right across the continent.
The Identity Movement began in France, but has since spread to Austria, Denmark and Germany. It appeals to a younger generation seeking to distance itself from the openly racist, xenophobic far-right parties of the past.
Sellner insists that the movement is not racist but "ethno-pluralist" -- it doesn't believe that white Christian culture is superior, he says, but that globalization is forcing countries to adopt a multiculturalism that destroys the ethnic and cultural values of individual states.
"We are not fighting against diversity. We don't think that people need to be racially pure," he says, adding that the group rejects "anti-Semitism or (anything) that identifies the problem with a certain people."
Yet the Identity Movement does single out one group: Muslims. Only an estimated 7% of Austrians are Muslim. But in Vienna and its suburbs -- where Sellner grew up -- Muslims make up more than 12% of the population, and some neighborhoods are majority Muslim.
Sellner claims this is "Islamization" and that the Muslim religion is incompatible with Austrian values -- but even he admits Islam has a long history in Vienna. "After all," he says, gesturing to the mirrored salon of the Viennese cafe we are talking in. "We are drinking coffee brought by the Turks!"
Given Europe's history, its 20th-century slide into fascism, the great wars that killed millions and singled out Jews and other minorities for genocide, isn't Sellner worried that the Identity Movement may be one step on a slippery slope to repeating that dark past?
No, he says: "I feel a kind of fatherly responsibility for young Austrian patriots not to let them go into these extreme subcultures that are driven by hate and crazy ideologies."
Instead he claims, perhaps counterintuitively, "I think the entire movement is the strongest force against far-right extremism in Austria."
It's difficult to know just how many support the Identity Movement, but Sellner hopes far-right anti-immigrant candidate Norbert Hofer will be voted into power in Sunday's presidential election.
And while that may not change Austrian policy immediately -- the Austrian president's role is largely ceremonial -- it would be proof of what Sellner believes is a "silent majority" in the country who share his beliefs.
"The main idea of equality and opportunity is great. No one would object to that. But the problem is egalitarianism. People are different. We look different. We have different cultures, languages and traditions. Politics and policies need to reflect that," he says.
"I think every society has only a certain capacity for integration. I think we have gone far beyond that capacity in Europe."