What organization's rally in Hollister in 1947 led to the famous motorcycle riot?

The organization whose 1947 Hollister rally led to the famous motorcycle riot was the American Motorcycle Association, formally the American Motorcyclist Association (AMA).

The event was the AMA-sanctioned Gypsy Tour, held in Hollister, California, from July 3 to 6, 1947. Motorcycle racing, socializing, and celebrations had taken place there before World War II, but the postwar revival drew far more people than the small town could handle. Estimates put attendance at roughly 2,000 to 4,000 people, including about 750 motorcyclists.

Some riders drank heavily, raced through streets, damaged property, and overwhelmed Hollister’s seven-person police force. About 50 people were arrested and roughly 60 injuries were reported, although the damage was limited. Newspapers and Life magazine dramatically portrayed the episode as a town being seized by bikers.

A common mix-up is confusing the AMA with the motorcycle clubs present, such as the Boozefighters and Pissed Off Bastards of Bloomington. The AMA sanctioned the rally; it was not itself one of those clubs. The episode later helped inspire the outlaw-biker image and the 1953 film The Wild One.