Tension between the two teams was always high as they consistently placed at the top of the Yugoslavfootball league and often won the national championships. In 1990, this fierce rivalry was heightened due to rising ethnic tensions in Yugoslavia. The first multi-party elections were held in most of Yugoslavia and communism was ousted in favour of more national-oriented parties. The second round of voting in Croatia was held on 6 May when the Croatian Democratic Union won under Franjo Tuđman. Slovenia and Croatia, under new leadership, were the leading forces behind a drive to reorganize Yugoslavia into a confederation, but were opposed by Serbia under Slobodan Milošević and the still powerful Communist system at state-level. Approximately 3,000 Delije made the trip to Zagreb. They were led by Željko Ražnatović, a Serbian nationalist who led the Serb Volunteer Guard. Between 15,000 and 20,000 spectators were estimated to have attended the game.
The clash
Up to several hours before the kickoff, skirmishes were reported around town between Dinamo and Red Star fans, which then carried over to the stadium. Provoked by stones reportedly being thrown at them by the Bad Blue Boys, Delije, situated in the stadium's segregated area reserved for visiting fans, began to tear up the advertising plates and eventually made their way to Dinamo fans, attacking them with torn-off seats and knives, while reportedly chanting Serbian nationalist slogans like "Zagreb is Serbian" and "We'll kill Tuđman".. Revolted by what they saw, The Bad Blue Boys at the north and east stands attempted to storm the field, but were quickly pushed back by the police who used batons and tear gas on them. Within minutes the situation got out of hand as the BBB could no longer be held back. They breached the fence and took to the field to charge at their Serbian counterparts. The police were quickly outnumbered and overwhelmed but came back with reinforcements, using armored vans and water cannons to disperse the rioters. More than an hour later, with hundreds injured, the riot was brought under control. The police were later deemed to have been too lenient with Delije during their early sporadic outbursts, which sparked the larger riot.
Zvonimir Boban's kick
Amidst the chaos several Dinamo players still remained on the field, the Red Star players having already left for the locker rooms. Zvonimir Boban, the Dinamo captain, kicked a police officer, Refik Ahmetović, who was allegedly mistreating a Dinamo supporter. The Bad Blue Boys came to Boban's defence, acting as human shield. The event would later come to symbolize in the minds of Croat nationalists the beginning of Croatian resistance against Serbian aggression. Boban was proclaimed a national hero of Croatia, but also attained a Croatian nationalist reputation in Serbia. He was suspended by the Yugoslav Football Association for six months, missing the 1990 World Cup, and had criminal charges filed against him. The officer he had attacked publicly forgave him for his actions several years later.
Aftermath
The riot marked the beginning of the end for the Yugoslav First League. After playing the entire following 1990–91 season, clubs from Slovenia and Croatia withdrew from competition. The league lasted for one more season after that as by the end of 1991–92 season the country disintegrated.
