A familiar site in Argentine soccer: riot police keep the peace outside Buenos Aires's River Plate Stadium. (Natacha Pisarenko/Associated Press)

The passion generated by an Argentine soccer crowd is a marvel to behold.

Giant flags and banners are unfurled on the stadium's terraces, flares and firecrackers are lit, drummers thump out a hypnotic beat, trumpeters toot their horns, and songs are sung by rabid fans who jump up and down in perfect syncopation for the entire duration of the game.

It is a colourful tapestry of sight and sound that is only overshadowed by the rich pageantry of skill and athleticism displayed on the field.

Sadly, the country that routinely stages such lively spectacles is also the same country responsible for infamous hooligan groups known as the Barras Bravas.

This year's Argentine Apertura campaign has been one of the most thrilling in a long time. Buenos Aires club Boca Juniors sits atop the league standings with a precarious three-point cushion over Estudiantes de La Plata heading into this weekend's final slate of games.

Boca can wrap up the title by earning a draw on Sunday but if they lose and Estudiantes wins its home game, a winner-take-all playoff between the teams at a neutral venue later this month will decide the championship.

It's all very exciting stuff, but what has been a spellbinding and entertaining race to the finish line has been eclipsed by countless acts of fan violence and hooliganism since the start of the season in August.

Like their European counterparts, the Ultras, Barras Bravas are organised groups of fans who forge direct relationships with clubs and players. The more hardcore sects often turn violent, using extortionate tactics that would make any mafia boss proud.

Barras Bravas, having been tolerated by the clubs for years, exploit their relationship by demanding the team to provide them with free tickets and transportation to away games. If the club doesn't oblige, the Barras Bravas threaten violence or issue death threats.

It's one of the worst kept secrets in Argentine soccer that notorious Barras Bravas leaders show up at a team's training sessions, where they convince players to "donate" a portion of their wages to them. Not surprisingly, they often target young players who feel they have no choice but to pay this protection money, thus guaranteeing their safety, and that of their family.

Even more disturbing are allegations that this is sometimes conducted with the blessing of the teams - the players' donations are used to finance the travelling expenses of the Barras Bravas. Some club directors have also been known to use the Barras Bravas as muscle in order to intimidate opponents or motivate one of their own players who is under-performing.

Riots in the stands and outside stadiums between warring factions of Barras Bravas forced the abandonment or suspension of no less than six games this season.

The Argentine Football Association finally took action in mid-November when it banned non-season ticket holders from attending games for the rest of the campaign in an attempt to stop the madness. Less than one week later the AFA did an amazing about-face thanks to a well co-ordinated stunt between two factions of Barras Bravas.

The Buenos Aires provincial government ordered a Racing Club-San Lorenzo match to be played in an empty stadium in La Plata, 60 kilometres from Buenos Aires, after rioting Racing fans forced the previous week's match against Independiente to be suspended in the second half.

Racing and San Lorenzo fans simultaneously picketed their teams' headquarters and stopped the team buses from leaving for La Plata. The plan worked like a charm - not only was the Racing Club-San Lorenzo match postponed, but the AFA rescinded its ban.

Even when suspended games eventually resume, the Barras Bravas have allegedly influenced the outcome.

A Sept. 10 match between mid-table Gimnasia La Plata and league-leaders Boca was abandoned at halftime when the referee claimed he was threatened in his dressing room by Gimnasia's president. The game was completed in November and saw Boca score four goals in 45 minutes to defeat Gimnasia 4-1 and open a four-point lead over Estudiantes.

The next day, public prosecutors launched an investigation into allegations that Barras Bravas made death threats to the Gimnasia players, demanding they throw the game against Boca in order to damage the title hopes of Estudiantes, the club's bitter rivals.

None of the Gimnasia players confirmed the allegations, but most Argentine commentators believe that's only because they feared retribution from the Barras Bravas.

So what can be done to curb the rising tide of violence in Argentina's soccer stadiums? Opinions vary, but most Argentines agree it's a problem best addressed by the AFA.

They might want to follow the example of Italian soccer officials who have given the referees the power to call off games at the first sign of trouble from the Ultras. Italy also levies clubs with fines and forces them to play future games at neutral venues behind close doors whenever violence erupts inside the stadium.

To be sure, the Barras Bravas are holding Argentine soccer for ransom, but the clubs are just as much to blame because they have tolerated - and in some cases fostered a relationship with these thugs - instead of seeing them for what they are: common criminals.