Poles, many of whom had traveled down by train or plane, outnumbered their rivals in the early afternoon and were by far the most vocal, chanting "Polska! Polska!" non-stop and "Deutschland, Deutschland, Auf Wiedersehen" at any passing German fan. But the atmosphere was still playful despite feared violence between the two camps.
"Germany is always an enemy of Poland, in our heart and in our soul, for historical reasons," said Andrzej, 39, as he enjoyed some food after stepping off the night train from Warsaw.
"Tonight will be treated like a war," he added, his smile however belying his words.
Some fighting broke out briefly but confrontations on Sunday afternoon were otherwise limited to chanting matches from one beer stand to the next and energetic flag waving.
A black, red and gold open-top double decker bus blared German World Cup songs from loudspeakers to drown out Polish chants, only to find its path blocked by Poles laying down in front of it while friends took pictures.
In the public viewing zone, German and Poles improvised a table football game.
"The atmosphere is fantastic," said Maria Helbing, 20, from Thuringia, in central Germany, as she watched two groups of fans trying to outdo each other vocally. "We Germans are bad singers. The Poles are currently better, we have to improve," she said.
Police were increasingly visible as the day went on however, following the parading fans and filming particularly loud groups. Some 400 German police officers, on hand to help their Austrian counterparts, were in charge of the city centre around the "fan mile" and public viewing zone, while some 25 Polish officers provided a familiar presence to their countrymen.
In total, some 25,000 German fans and 20,000 Poles were expected in Klagenfurt on Sunday, prompting fears of violence between these traditional rivals.
Saturday evening, some fighting resulted in 16 arrests. But police quickly intervened and brought the situation was brought under control before it could escalate, said Klagenfurt police spokesman Gottlieb Tuerk.
Maciej Bruszewski, who drove down from Nidzica in northern Poland with three friends, downplayed fears however.
"We're very peaceful: two years ago during the World Cup in Germany, there was also a game between Germany and Poland and nothing happened," he said. "And nothing will happen tonight. If we win, we'll be drinking until morning. If we lose, we're leaving right after the match."
German and Austrian police officers could be seen posing or chatting with fans, while supporters from the opposing camps took pictures of each other. Flags and team shirts were the accessory of choice, but a group of Scotsmen in kilts - "we hoped Scotland would qualify so we booked early" - and Germans in lederhosen could also be seen in the sea of red, white, black and gold.
A group of 12 young men from Baden Wuerttemberg downed beers, all dressed in full German kit, from shirt to socks. "After the World Cup and the great atmosphere there, we decided we must come to Austria and Switzerland to experience this great feeling again," said Armin Thieringer, describing the mood as "perfect, pure emotion."
As part of the German football association fan club, he and his friends were lucky to have scored tickets to each of their country's group stage matches. Others still walked the streets with pieces of cardboard saying "looking for tickets" in various languages.
Daniel, 23, travelled to Klagenfurt from Bielefeld, in western Germany, and was confident he would see Sunday's game. "You just need to invest a little money but there are still people who have extra tickets," he said, standing on a street corner with a sign reading "need a ticket."
Even without a ticket, there is fun to be had, added Matthias Stein, from the German "fan embassy" group. "It's still worth making the trip even if you have no tickets, just to experience the atmosphere, to be part of the event," he said.