It's happened again. On Sunday, more than 20 Egyptian soccer fans were killed in an altercation with security forces outside a stadium that was hosting a game between Cairo clubs Zamalek and ENPPI. The tragic incident comes three years after the deaths of 72 Al Ahly soccer fans during a match in the city of Port Said, one of the worst disasters in the history of the sport.

As they did three years ago, Egyptian authorities have suspended the league in the wake of the violence. Many of the dead reportedly belong to a hard-core group of Zamalek supporters known as the Zamalek White Knights. Egyptian officials say police fired tear gas to disperse a crowd of ticketless fans attempting to force its way into the stadium. Public prosecutors have ordered the arrest of leading Zamalek White Knights members.

Clashes between soccer fans and security forces outside an Egyptian football match leave at least 25 people dead. (Reuters)

On their Facebook page, the White Knights accuse security forces of carrying out "a planned massacre." One prominent White Knights leader wrote in an online post that authorities had let some 5,000 fans enter a narrow enclosure and then shut them in, proceeding to lob rounds of tear gas into the massed crowd.

"People died from the gas and the pushing, but they had shut the exit," wrote the leader, who goes by the online alias Yassir Miracle. He added that security forces "were searching the corpses and the people who had just collapsed and collecting the wallets and money.”

The enmity between the Egyptian state and the country's ultras -- organized, hardened groups of passionate fans, such as the White Knights -- runs deep. In the 2011 revolution that overthrew the dictatorship of Hosni Mubarak, ultras from both Zamalek and arch-rival Al Ahly had declared a truce and formed the front-ranks of the protesters in Tahrir Square, manning its battle lines and clashing with security forces and pro-Mubarak thugs.

Well before Mubarak's ouster, ultras chanted anti-regime songs at stadiums, which became spaces for angry displays of dissent in what was an otherwise stifling, authoritarian environment. The fanaticism an ultra has for his team and the sense of solidarity he shares with his fellow fans lends itself well to political mobilization.

One Ahlawy ultra -- a fan of Al Ahly -- explained to a GQ reporter the vital role the ultras played in enabling the revolution:

"The whole concept of any independent ­organisation didn't exist, no unions, no political parties. Nothing was organized. And then we started to organize football ultras," the ultra, named Assad, said. "They feared us. It wasn't just supporting a team; we were fighting the police, fighting the government, fighting for our rights. Of course, I don't want to say we were solely responsible for bringing down Mubarak," he told GQ.

"But our role was to make people dream, letting them know if a cop hits you, you can hit them back. During the revolution, there was the Muslim Brotherhood, the activists and us, the Ultras. That's it."

The gruesome 2012 disaster in Port Said when crowd violence and conspicuously poor policing led to 72 fans, many of them Ahlawy members, dying, highlighted the tension between these groups and the state. At the time, Al Ahly's ultras issued a statement saying the incident showed how Egypt's interim military rulers wanted "to punish us and execute us for our participation in the revolution against suppression."

A government-ordered ban on fans attending matches was just lifted this year, but it's likely to be reimposed after the latest violence.

There are differing views on the politics of Egypt's ultras, which can be loosely described as anti-establishment if not outright anarchist. Many come from humble, working-class origins but not all.

Like their equivalents in Europe, the ultras are often described in the media and by figures in power as hooligans, looking to stir senseless trouble. Some regime loyalists have attempted to paint them as a front for the now-outlawed Muslim Brotherhood -- a claim that speaks less of the soccer fans' Islamist leanings than it does of the climate of repression currently in place in Egypt.

Al Ahly and Zamalek are Egypt's two biggest teams and the clubs are veritable national institutions; their top executives often are closely linked with the country's governing status quo. Zamalek President Mortada Mansour, a Mubarak-era official now allied with Egypt's President Abdel Fattah al-Sissi, has spent much of the past year trying to get the White Knights listed as a terror organization.

White Knight ultras had stormed Zamalek's club last August, calling on Mansour's resignation. Twenty members of the group were sentenced to several years in prison last month. The group has labeled Mansour "the regime's dog."

Egypt is hardly the only country with a culture of ultras; the Egyptian strain has largely been borrowed from countries like Italy and Serbia. Ultra groups exist across southern and eastern Europe, sometimes representing the far-fringes of the political spectrum.

But amid Egypt's polarized politics, where the powers-that-be have been cracking down on dissent, the die-hard ultras play an increasingly rare role--defiantly standing up to authority. As the tragic scenes this weekend make all too clear, soccer in Egypt is about much more than just a game.