By Matthew Davis
BBC News Online, Athens
A Puma T-shirt draws suspicious looks
A burly security guard eyes my Puma T-shirt with suspicion as I am about to enter the Olympic judo hall.
"That is not an official sponsor, I trust you are not trying to ambush us?" he says.
He has a hit list of restricted items for spectators - just one of an array of measures Athens has taken to protect the 11 main Olympic brands like Coke, Kodak and Adidas.
Each has paid about $60m to associate with the Games and none want unofficial advertisers stealing their thunder.
Perhaps the greatest coup in "ambush marketing" came at in Atlanta 1996, when Nike bought a vast number of billboards around Olympic sites, thumbing their noses at Adidas.
Brand consultants say such tactics represent one of the biggest threats to the future of major sports events because they strike at the deals that finance them.
Athens 2004 and the International Olympic Committee are taking measures big and small to combat them.
Flags, umbrellas, shirts, hats bags with trademarks of rival sponsors
Electronic equipment of non-rights holding broadcasters
Flags of non-participating countries
Banners larger than 1m square
Pirate Athens 2004 products
Food - except for proven medical reasons
Unauthorised entry of TV presenters
A row of people all wearing the same logo at an Olympic venue - hoping to get on television - might be ambush marketing on a small scale.
So hats, bags, umbrellas - and T-shirts - with distinctive trademarks of sponsors' competitors are "restricted".
So too is food - especially if it is not an official Big Mac. In searing heat, spectators are also being asked to leave "unlicensed" water bottles.
Also out are flags from non-participating countries, or buying a ticket with anything other than a Visa card.
With a television audience of four billion in 220 countries watching 3,800 hours of broadcast coverage, the Olympics are as revered by sponsors as they are by athletes and spectators.
In the run-up to Athens 2004, an estimated $750,000 was spent on clearing thousands of billboards from buildings and rooftops around Athens - and reserving those near stadiums for the main sponsors.
In central Athens, many buildings are draped in huge advertisements for the select brands - especially Adidas.
Sponsors are doing brisk business
Industry experts say it is the purity of the Olympic name that makes it so attractive a target.
John Allert, head of consulting at Interbrand, told the BBC: "Event marketers are taking increasingly aggressive steps to try to stamp out ambushing - but with limited success.
"Nike was mentioned almost as frequently in Euro 2004 as Adidas - when Adidas was spending a fortune as official sponsor.
Threat to Games
"These tactics cut to the heart of the commercial viability of the Games, and represents one of their single biggest threats.
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
Without guaranteeing exclusivity, it is harder to play competitive sponsors off against each other."
Gerhard Heiberg, head of the IOC's Marketing Commission, said the IOC worked hard to protect the values in the Olympic Charter.
He said the Olympics were "unique for being the only major sports event that does not carry advertising in arenas or on athletes clothing".
"We support the clean venues policy, of course, but the individual measures are down to the host nation," he told the BBC.
The Athens organising committee said individuals were unlikely to be stopped for wearing the wrong shirt.
The measures were aimed at "groups drawing attention to themselves" they said.
Spectators like Terry Faulkner are suffering in the heat
On day one of competition at the main Olympic complex, staff appeared a little on edge.
Workers were handing out free Kodak cameras near a prefabricated McDonalds, which was doing brisk business.
"There are a lot of rules," said one. "You can take our picture, but please, no names."
Terry Faulkner - supping an official mineral water - had come to see his son Adam, 22, compete in the swimming.
"There's no roof on the stadium, it's so hot. I only flew in yesterday, and I am suffering."