Aides to the self-described democratic socialist had originally booked a Phoenix theater that could accommodate fewer than half the number of people who turned out. The crowd estimate of more than 11,000 people was provided by staff at the convention center, where Sanders also appeared Saturday at a convention of progressive activists.
"Somebody told me people are giving up on the political process," Sanders said as he greeted the crowd Saturday night. "Not what I see here tonight."
He delivered largely the same stump speech he has been for months, lamenting the level of income inequality in the country and railing against the outsized political influence of the "billionaire class."
"Their greed has got to end, and we are going to end it for them," Sanders said, drawing chants of "Bernie, Bernie, Bernie."
While few Democratic elites give the 73-year-old Sanders much chance of winning the nomination, his campaign is being taken increasing seriously by the Clinton camp.
Saturday night's crowd -- in a reliably red state -- surpassed Sanders's previous largest draw, a few weeks ago in Madison, Wis., where close to 10,000 people turned out.
Among those in the audience Saturday night was Amanda Appelbaum, a stay-at-home mom in Phoenix with a baby on the way who wore a T-shirt proclaiming "Bernie Effin Sanders 2016."
"In the last 10 years, I've felt I've had no reason to vote or focus on politics," Appelbaum, 29, said, saying Sanders was different. "I love that he doesn't take corporate donations, that it's all about being elected by the people."
Brett Barry, 50, a real-estate broker from Scottsdale, said Sanders drew such a large audience because "we're hungry here."
"Some of our liberal friends have given up because they don't think [Sanders] has a realistic chance," Barry said. "I get tired of hearing that."