About 100 sweating Olympic pin collectors braved temperatures higher than 35 C to trade their stockpiles with fellow hobbyists at the opening of the Coca-Cola Olympic venue Sunday.

Pin collecting has gained international recognition as an "unofficial Olympic sport", and the event attracted traders and spectators from every corner of China and the globe.

"It's a Sunday, and the Games hasn't even started yet. It'll only get better and better," American Bud Kling, the center's head pin coordinator and zealous pin-collecting enthusiast, said.

Merritt Canfield and Jeannie Quaintance, a couple who had been trading Olympic pins since Atlanta 1996, said the hobby provided an incentive to travel, as the Games "always brings out the best of each host city".

"These pins speak a universal language, and it's a great way to get to know people," said Quaintance, who owns nearly 7,000 pins.

A few meters away sat retired public servant Xie Jinghua, who has invested nearly two decades in the pastime. Her genre of choice is National Olympic Committee (NOC) pins, and she has amassed about 150 of the variety.

"The thing about these pins is they're never for sale and could only be obtained through swapping, and mostly in Continental and intercontinental Games," she said.

While some seasoned collectors were questing for very particular pins, others were just dipping their toes in the pastime.

American Noah was a newbie to both China and the hobby.

"I'm just starting to learn about it all," he said.

The tradition of Games pin trading began at the first modern Olympics in Athens, Greece in 1896. In 1988, the International Olympic Committee authorized Coca-Cola to run the first official pin-trading center at the Winter Games in Calgary, Canada.

"Olympic pins themselves have become an integral part of Olympic culture, and the pin trading center is where people communicate during the Olympic Games," David Brooks, vice-president of Coca-Cola (China) and head of its Olympic Project Group, said.

"We hope that the pin-trading centers ... offer the ultimate experience and memories of the Olympics and culture for people from all around the world," he said.

The Coca-Cola Shuang Zone, a pin-trading center in Chaoyang Park, will be open to public from Aug 1 to 24.