"Yes!" many of them screamed, while others pumped fists in the air, some of them clad in the red-and-white checkered jersey symbolizing Croatia's historic coat-of-arms.
A chance visitor would be forgiven for thinking that he or she was in the middle of the soccer-mad Croatian capital of Zagreb.
Actually, this was happening in the center of Belgrade, the capital of Serbia, Croatia's "arch-enemy" in sports, politics and -- two decades ago -- on the battlefield as the former Yugoslavia collapsed in a series of wars.
Football rivalries, sometimes reflecting historical relations, often run very deep -- just ask your average English, Irish, or Scottish fan.
In the light of recent bloody history, however, they are taken to the extreme in the Balkans. Deep antagonism, even hatred, especially among the young, seemed to be the rule and supporting the "others" is anathema.
But for several dozen young Belgrade residents, as well as a growing number of people throughout the region, this attitude just does not make sense at all.
That's why big-stage debutants Bosnia and relative tournament veterans Croatia, the only teams from the region in Brazil, can count on noisy support from some unlikely quarters this time around.
"I can only say that I cheer from the bottom of my heart for the neighbors, a little bit more for Bosnia than for Croatia, I have to say, but tonight for Croatia," a woman in her twenties told RFE/RL's Balkan service in a cultural center in Belgrade to the sound of almost frantic cheering for Croatia in the background.
The shared viewing of this and other games, under the slogan "Cheer for Your Neighbor," was organized by the "Youth Initiative for Human Rights,"a Belgrade-based non-governmental organization with a reputation for breaking ethnic-based taboos in Serbian society.
"We have to support each other. After all, we belong to the same culture," another woman said, standing against a wall adorned by the Serbian, Croatian, and Bosnian flags.
One of those who could be credited with helping this thaw is Novak Djokovic, the world's number two tennis player.
Util recently, he was seen outside of Serbia as someone who allows himself to be too easily used by politicians when it comes promoting a nationalist Serbian identity.
But, after massive floods hit Bosnia, Serbia and Croatia last month, "Nole" did not spurn the opportunity to appeal for assistance to the three countries, saying that, when disaster strikes, there is no difference between these peoples.
He also didn't do any harm by saying that, in the absence of Serbia at the World Cup, he would cheer for Bosnia and Croatia.
Still, this may not be enough to persuade most Bosnian Serbs and Croats to support "the Dragons" as Bosnia's national team is affectionately known.
The team has been dominated by Bosniak Muslim players, but over the two decades since it first came into existence, Bosnian Serb and Croat coaches and players have played very important roles.
The western part of the ethnically divided city of Mostar looks no different than most Croatian cities these days -- Croatian flags and banners in support of "the Fiery Ones" adorn the town's streets and buildings.
Unlike Sarajevo, which awaits Bosnia's first game against Argentina on June 15 in a state of euphoria, in Banja Luka, the capital of Bosnia's Serb Republic, the World Cup does not seem to arouse any passion -- apart from those who say that, in the absence of Serbia, they would support any team except Bosnia and Croatia.
But even there, after years of widespread open contempt for the Bosnian team, there are signs that things are changing.
"I will support Bosnia, why not? After all, our players from Banja Luka and from Republika Srpska are on the team," one young man told RFE/RL.
-- Written by Nedim Dervisbegovic in Prague based on reporting by Zoran Glavonjic in Belgrade, Selma Boracic in Sarajevo, and Erduan Katana in Banja Luka