The Olympic fencing competition opened here last Saturday with gleaming foils, impeccable white uniforms – and screaming. Also foot-stomping, chanting, booing and some catchy songs. Fencing matches are normally conducted against a background of respectful silence from the crowd, but no one passed that message to the Brazilians in the stands, few of whom, it seemed, had ever seen fencing before.

"When we first showed up, the volume of cheering was pleasantly surprising – it was amazing, really," said Brad Goldie, president of the Canadian Fencing Federation. "But the first time they booed … You never hear booing at a fencing event."

In the first five days of the Summer Games, Olympic sporting tradition has run smack into a Brazilian fan culture forged in soccer stadiums. At every sport being played here, there is chanting, singing and vociferous booing from the crowd.

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The spectators passionately defend the underdog (even when, especially when, that underdog is a country half the fans seem never to have heard of) – and they will suddenly and spontaneously unite around a player or a team they love, or loathe, for reasons that can leave athletes themselves bewildered.

The response from competitors and fans from other countries has ranged from approbation to mystification to an if-you-can't-beat-'em ramping up of noise. Some athletes have grumbled to the media about being thrown off their game, and announcers have made increasingly desperate pleas for decorum.

But Martin Curi, a German anthropologist who has studied Brazilian spectator behaviour for more than a decade, wonders what else anyone was expecting from the Games in Rio.

"The Olympic Committee wants to make the Olympics grow in South America, so they came to Brazil – well, if you're in Brazil, you get Brazilians," he said.

"Brazil is not really an Olympic country, it's a soccer country – so what are people used to? They're used to the logic of soccer and the behaviour of soccer, so most are showing the same behaviour as in a football stadium. So of course they defend their own athletes and their own teams and of course they make a lot of noise, sing and disagree with the referee."

Brazilians are particularly boisterous in judged sports such as gymnastics, where they give full force to their opinions about point tallies, he said, much as they hurl abuse at soccer referees awarding red cards.

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Mr. Goldie said he was glad Canada's fencers spent time in the arena before they competed, so that they weren't rattled when they picked up their sabres. "I think Brazilian fans recognized that we were surprised [by the booing] and some told me, 'Oh no, it's not that we hate you – we just don't like you scoring points on our guy.'"

When they don't have a Brazilian to support, hometown fans reliably cheer for Cuba, and for any small African country, or anyone they can construe as an underdog (which, ironically, is how most Brazilians see their own enormous country, Prof. Curi noted). Zimbabwe's women's soccer team was showered with noisy Brazilian love, as was the Fijian women's rugby sevens squad, whose players looked visibly startled by the waves of noise coming from stands where the Fijians were a tiny island of fans.

A male Qatari beach volleyball duo were cheered on by songs spontaneously revamped to include the name Qatar. A Latvian beach volleyball player got full crowd backing because one loud fan pointed out that his blond mullet hairstyle made him look like a Brazilian soccer star from the 1980s. Any Latin American country usually gets fan support, too – unless it's Argentina. Brazilians have carried their legendary soccer rivalry with the Argentines into every other sport here, resulting in booing at judo and handball. Police had to carry out fans who got into a brawl at an Olympic tennis match (this appears to be a first) when Argentine Juan Martin Del Potro was playing a Portuguese opponent, and was booed by Brazilians.

The only other target to come in for as much Brazilian abuse are athletes from the United States, who are reliably booed every time they pick up a ball to serve or a foil to fence. Even gymnastics phenomenon Simone Biles was booed by Brazil. This appears to be some combination of Brazilian hatred of perceived U.S. imperialism and just general dislike of anyone seen to be too strong.

"In football, it's absolutely normal to boo," said Prof. Curi, who teaches anthropology at the Federal University of Fluminense in Rio. "You pick a player you don't like for some reason." The target is often U.S. soccer goal keeper Hope Solo, who is still being punished for tweeting pictures of the mosquito net she said she was bringing to Rio to avoid infection with the Zika virus. The U.S. women's soccer team is also being subjected to homophobic chanting, Prof. Curi said, a fixture at soccer games here.

Rio native Danielle Pereira, 28, went to see Brazil play Argentina in volleyball. She booed, of course. "The game is a party and booing is a normal game thing," she said. "This is not disrespectful, this is part of our culture, it's for fun." But at the end of the game, the announcer asked the crowd to express their appreciation for both sides.

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"Fans applauded both the Brazilians and the Argentinians. Honestly, I thought people would be booing, but no. They had no problem with that. They showed respect for them." (It probably didn't hurt that Brazil won, 2-0.)

At fencing, the fan noise has kept competitors from being able to hear the referee's crucial call of "Allez!" and it has resulted in some false starts at swimming. And there is controversy in the world of table tennis, where players are complaining that the fans make it difficult for them to play their matches.

"It's really tough – the spectators start to sing, and it is tough to hear my racket: From the sound of my racket I know how strong I have to play," said Aleksandar Karakasevic, a Serbian competitor. "This is not, when you win a point, they scream. Many times I have played world championships like that. This is totally different. It was strange and difficult." He discussed the challenge with the International Table Tennis Federation, which in turn had announcers instruct the fans in appropriate table-tennis etiquette, and Mr. Karakasevic said the disruption has diminished as the days have passed.

Mr. Goldie of the fencing federation says it has been obvious that Brazilian competitors do better when the crowd noisily backs them, and it's been good for some other countries, too. In a match between the United States and France, the U.S. competitor was being booed – so U.S. fans got louder with support. That, in turn, drove the French to step it up. And after five days of entreaties from the announcers, the Brazilians are starting to get on board with fencing decorum.

"Our fans have become better fans because of being here with the Brazilians, watching how passionate they are and how they cheer – and the Brazilians have become much better. They're cheering hard for the Brazilians and kind of turning back into what we consider a normal fan for everyone else. I love it – it's making for a fantastic Olympics."

With a report from Elisângela Mendonça