Among those caught in the mayhem was defending champion Alberto Contador, who overcame another crash, banging his troublesome right knee after an early spill.
"I had a problem with my handlebars, which knocked into another rider's saddle," Contador said after safely completing the stage in 12th place. "It pushed me toward the spectators on the side, and I went into them and the bike hit me on the right knee again."
Sanchez moved into second place overall, followed by Australia's Cadel Evans.
Veteran Kazakh rider Alexandre Vinokourov fractured his right thigh bone and withdrew after crashing near the halfway point of the 129-mile route from Issoire to Saint-Flour in the Massif Central.
Spain's Juan Antonio Flecha was hit by a car later on, and took down Dutchman Johnny Hoogerland with him as he flew sideways off his saddle. Both got up and kept riding.
Voeckler once defended the yellow jersey for nine days during the 2004 Tour, but he could not match Sanchez.
"These are good times. I wouldn't have bet on taking the yellow jersey today," Voeckler said. "Time passes and I appreciate this one even more."