Wednesday's 16th stage of the Tour de France takes riders up the mythical L'Alpe d'Huez mountain over a 15.5-kilometer (9.6-mile) ascent lined with frenzied supporters.
"Oh God! It's Bikestock'' film star Robin Williams said, comparing it to the legendary Woodstock rock concert of the late 1960s that attracted hundreds of thousands of music fans.
Williams, star of "Good Will Hunting,'' "Good Morning Vietnam'' and other movies, now joins rock star Sheryl Crow, and Prince Albert of Monaco as the latest celebrity guest invited by Lance Armstrong during the three-week race.
Speaking after Armstrong won Tuesday's 15th stage to take the overall leader's yellow jersey, Williams could not hide his excitement about L'Alpe d'Huez.
"An uphill time trial, the crowds are wild,'' said Williams.
Asked if he'd like to ride the grueling stage himself, Williams added: "I'd love to ... if I could find a bike.''
An amateur cycling enthusiast, Williams also joined Armstrong during last year's Tour. This year, Armstrong is bidding to make history with a sixth straight Tour win.
A quiet Alpine retreat, the village of Alpe d'Huez has a population of 1,500. But cycling fans began flocking there 11 days ago to claim a top spot on the mountain.
Roads were expected to be blocked off at early Wednesday, but instead the decision was made to stop traffic at noon Tuesday, said Maroussia Perez-Perrot, press officer for the local town hall.
She said up to 1 million people were expected to come, compared to 800,000 last year.
Fans are camping out "in tents, in fields, on rocks,'' Perez-Perrot said. "It's very unusual.''