As many as 4,000 anti-fascist demonstrators gathered in Vienna late Friday, organisers said, to protest against a ball attended by European extreme-right figures including France's Marine Le Pen.
Accompanied by a large police presence, two groups of protestors marched along different routes and converged at Heldenplatz outside the Hofburg palace where the Wiener Korporationsring (WKR) ball was taking place.
Police, who numbered several hundred drawn in from several other regions of Austria, many in riot gear and with helicopters overhead, put the number of demonstrators at only 500, however.
Protestors, some of them from other countries, most notably Germany, chanted "Nazis raus!" ("Nazis out!") and held placards with photos of Holocaust victims and slogans reading "Don't dance on my grave."
The demonstrators were outraged that the ball was being held on January 27, which is Holocaust Memorial Day.
The roughly 3,000 guests to the ball were able to enter the Hofburg palace, the Habsburg dynasty's opulent former imperial residence, well away from the protests, however. The ball got under way, late, at 10:00 pm.
The WKR ball, part of the Austrian capital's famous ball season, is organised by traditional, often right-wing, Austrian student fraternities. It has always been controversial and often attracts protests.
But this year's event, the 59th, coincided with the anniversary of the liberation of the Nazi Auschwitz death camp and International Holocaust Remembrance Day.
"It is all the more regrettable and perfidious that today of all days, people will dance on the graves of Auschwitz," Eva Glawischnig, head of Austria's Green party, said at a Holocaust commemoration ceremony earlier.
Others attending include Heinz-Christian Strache, head of Austria's far-right Freedom Party (FPOe), who took Le Pen as his guest to the ball, a statement from her party issued in Paris said.
The statement also said that National Front head Le Pen, who is running for French president later this year, would also meet other leaders of "patriotic movements."
Other European extreme-right politicians scheduled to be present include Kent Ekeroth of the Sweden Democrats and Philip Claeys of the Flemish party Vlaams Belang, the Austria Press Agency reported.
In Germany, the extreme-right Republikaner party said that unlike in previous years its leader Rolf Schlierer could not attend this time, but that "numerous" other party members would be there.
"There is a certain uproar (about the ball) but this event is public, anyone can buy a ticket," a spokesman said.
The NPD, another German party, told AFP that five members would be in Vienna but declined to say whether they would be at the ball.
"People attending this ball are not extremists, as is always claimed," Strache told reporters. The party said it was a coincidence that the event was taking place on January 27.
The Austrian arm of the UN cultural organisation, UNESCO, last week struck the traditional Viennese ball, of which there are around 100, off its intangible cultural heritage list because of the WKR event.
The ball is however being held at the Hofburg for the last time after the authority responsible for the palace told the WKR in December to find another venue for 2013.
Strache, whose party has roughly equal support to the ruling Social Democrats of Chancellor Werner Faymann, according to polls, said the decision was a "capitulation to the bullying of the left and to street politics."