Exactly what happened in Port Said on Wednesday night, and exactly who was to blame for the 74 deaths, remains unclear. What is certain, though, is that football "ultras" have become increasingly visible – and not just in Egypt.

The ultras are, essentially, radical supporters' groups, but to dismiss them as hooligans is overly simplistic, underplaying both their level of organisation and what makes them so attractive to disaffected male youths in particular.

Nobody doubts that ultras played a significant role in the demonstrations in Tahrir Square. The question is which club's they were, with both Al Ahly and Zamalek claiming credit, and both claiming the other was the establishment side that enjoyed the patronage of Hosni Mubarak.

For a while a détente existed between the two teams as they rounded on a common enemy. "We fought the police in every match," said Ahmed, a leader of the Zamalek White Knights ultra group. "We know when they run. We taught the protesters how to throw bricks."

The situation is similar to that in Belgrade after the fall of Slobodan Milosevic, for which fans of Crvena Zvezda (Red Star) and Partizan both claimed credit. It was at Zvezda's Marakana stadium on 26 July 2000 that there came the first clear sign the Serbian public had turned against Milosevic, as fans at a Champions League qualifier against Torpedo Kutaisi of Georgia began chanting: "Do Serbia a favour, Slobodan, and kill yourself."

Police waded in, but this was the Zvezda hardcore, the Delije (the Strong Ones), and they fought back. They had been an anarchic hooligan force in the 80s, regularly causing havoc at away games, and Milosevic had sought to harness their power by appointing Arkan, later a notorious warlord, to control them.

In the eyes of the Delije, it was they who enacted the first battle of the war in the former Yugoslavia as they rioted in Zagreb in 1990 at a league game against Dinamo, whose Bad Blue Boys were ultras with a clear Croatian nationalist ideology. Both ultra groups later became military units during the war. Behind the main stand at the Maksimir stadium in Zagreb stands a memorial to the Bad Blue Boys who died at the front. That the Delije had turned against Milosevic was hugely significant.

Most games turned into anti-Milosevic rallies, with the police powerless to act. Civil disobedience spread and, when the mayor of Cacak, Velja Iljic, led his decisive march on Belgrade, his column of 10,000 protesters was joined in the capital by two groups: the students and the Delije (and, possibly, the Partizan equivalent, the Grobari, or Gravediggers). They were independent of the state, organised, had a clearly defined leadership structure and were used to fighting the police.

The ultra groups are not just about fighting. A few years ago, I had lunch with a number of Delije leaders. The atmosphere was reminiscent of a mafia film, with a clear order of seniority behind the bonhomie and constant interruptions as those lower down the chain sought orders and favours. "We look after our own, you know?" one said, explaining how the Delije found jobs for their members and supported those too ill to work. I later saw the Delije helping a man all but crippled by polio into the stadium for a game. It's easy to understand the group's appeal.

More recently, Serbian ultras caused the abandonment of a Euro 2012 qualifier against Italy in Genoa, cutting down fences and firing flares. The reasons offered ran a typically wide gamut and suggested how difficult ultras are to categorise: some said they were highlighting their opposition to US policy on Kosovo as Hillary Clinton visited Belgrade; some said they were protesting about the arrest of a Serbian drug-dealer; some were angry about the transfer of the former Zvezda goalkeeper Vladimir Stojkovic to Partizan.

Football violence has been around almost as long as football, but an early instance of fans becoming politicised came in Budapest in the 1920s. As an economy ravaged by the first world war boomed, many from the working-class neighbourhoods of Pest felt they were being left behind.

As the historian Tamás Krausz put it, "foreign money, ie 'Jewish capital' and 'foreigners' in general, was considered the main obstacle in the way of Hungarian prosperity". The disaffected found in Ferencvaros a football club that offered a sense of identity and purpose they could not find elsewhere, and accordingly it became a rallying point for anti-Semitism and ultimately the Arrow Cross party that led Hungary into an alliance with Nazi Germany.

It would be wrong to speak of ultras as a movement, for the outlook and ideology of groups varies radically from country to country, from club to club and even within clubs. Some groups are concerned with nothing more than choreography, organising spectacular displays of support. Some just enjoy violence. I still remember the excitement that glinted in the eyes of a Spartak Moscow ultra who stared at me and said: "But the best thing is when you go abroad and fight the police … like a war."

Some are essentially criminal gangs, involved in smuggling and drug-running. But some groups have become politically active. Last year, for instance, a few minutes into the opening game of the Copa America in Argentina, where internecine conflict between various barras (as the ultra groups are known there) has claimed 157 lives since 1979, one group of home fans unveiled a banner in support of a candidate in the Buenos Aires provincial elections, prompting a furious reaction from elsewhere in the ground.

The barras often seem like mobs for hire, offering support to whoever gives them the best deal, but elsewhere there are more clearly defined political goals. The ultra groups are rarely ideologically coherent and are prone to abrupt political shifts, making them dangerous allies, But in street demonstrations, the ease with which they can be mobilised and their willingness to fight has made them an increasingly significant force.