By Laura Smith-Spark
BBC News, Stuttgart
Dutch fans were forced to surrender trousers and other branded items
The streets around Stuttgart's Schlossplatz are filled with colour ahead of the clash between Spain and Tunisia - some fans dressed in the red and yellow of Spain, others in Tunisia's red and white.
But what stadium officials are looking at is not which team people are supporting - but what company made their shirt.
Up to 1,000 Dutch fans watched their side play Ivory Coast in their underpants on Friday after they were denied entry to Stuttgart's stadium for wearing orange trousers with the name of a Dutch brewery which was not an official sponsor.
Faced with missing the game or ditching their orange lederhosen - given away by the brewery - they made the obvious choice.
Fifa officials said the trousers were an attempt at so-called ambush marketing - where a company tries to gain free publicity - and that they had to act to protect the interests of sponsors.
American firm Anheuser Busch, which makes Budweiser beer, is among 15 major companies to have paid up to $50m (£27m, 40m euros) each for the right to be official partners at this World Cup.
The tournament's official sponsors want their rights protected
"Anyone can wear whatever they want but, if a company tries to carry out ambush marketing, Fifa must prevent that happening," Fifa communications director Markus Siegler told reporters.
"In common with the IOC (International Olympic Committee) and Uefa, we do not tell individual supporters what to wear, but I will remind you that Fifa has already won a court case against a beer manufacturer who tried this sort of thing."
Supporters in any of Germany's 12 host cities can hardly fail to notice who the official sponsors are.
Coca Cola banners cover the wire fence of the Fan Fest public viewing area in Stuttgart, while car-maker Hyundai shows off its latest models at a stand by the entrance and the Mastercard logo is prominently displayed.
The Fifa Fan Shop sells the official Adidas replica kit - and at 60 to 65 euros for a shirt it may prove too expensive for some.
'Not about football'
Some supporters argue that the efforts to protect the official partners' commercial interests means the fans lose out.
1984: Kodak sponsors TV broadcasts, despite Fuji being Olympics' official sponsor. Fuji returns favour at Seoul 1988 Games
1992: Nike sponsors news conferences with the US basketball team. Michael Jordan accepts the gold medal for basketball and covers up his Reebok logo
1994: American Express creates runs ads claiming Americans do not need "Visas" to travel to Norway (for Winter Olympics)
1996: Nike buys out billboards around Olympic sites
2000: Qantas Airlines' slogan "Spirit of Australia" coincidentally sounds like games slogan "Share the spirit" to chagrin of official sponsor Ansett Air
Cristina Morante, who has come from Asturias to cheer on Spain, says she has opted to buy an unofficial 10 euro replica top because of the cost of a real one.
"We would like to take our own drinks into the Fan Fest but we cannot because they only sell the drinks they want to sell. The beers are very expensive in there," she adds.
Ramze Maamer, a Tunisian living in Stuttgart, says: "It's too much. It's not about football, it's just a marketing thing, the World Cup.
"If we have paid for tickets that should be enough. We are really hoping that in South Africa (in 2010) it will be different. The sponsor companies already have all the tickets, and not the fans."
Mexico fan Rudy Magallon, who has travelled from Los Angeles for the tournament, says he understands why the official partners want to prevent other firms grabbing free publicity.
But, he says, making fans take off their trousers is going too far.
'Much at stake'
"It's an embarrassment. I think it would make me feel unwanted," he says.
One of the highest profile brand rivalries is between Adidas and Nike
Viken Oijizmedjian, a Fifa spokesman in Stuttgart, told the BBC News website that individual fans need not worry because the regulations on what brands can be worn apply chiefly to players and officials.
"Individual supporters can wear what they want. If they come in their normal tracksuit, that's okay," he said.
"But if companies are trying to do ambush marketing, that is not allowed because it can be seen on television."
With so much at stake financially for the organisers and sponsors of major sporting events, it seems unlikely the rules will be any less strict in the future.
In fact, the organisers of the London 2012 Olympic Games have already listed a string of Olympic-related words and images that are off limits to all but official sponsors.
And mindful of the 1996 Atlanta Olympics - when Nike "ambushed" sponsors Adidas by buying up vast numbers of billboards around the Olympic sites - the London committee has already taken the precaution of booking almost all the city's billboard space during the games.
Marketing 'very important'
For Tunisian Sahbani Anis, visiting Stuttgart from Paris, the economic advantages an event like the World cup brings to host countries outweigh the restrictions imposed under sponsorship deals.
"There's a lot of marketing but I think it's very important because it is the reason why countries seek to organise the World Cup," he says.
"Without the marketing people would not go to the shops, buy the goods and so allow the economy of that country to do well.
"For example, next time the host will be South Africa and it will have the chance to relaunch its economy.
"Football is perhaps a way to get money into an African country it would not see otherwise. I hope we will see that in South Africa."