At least 5,500 Coloradans crammed into a Denver gymnasium, an adjacent atrium, and lacrosse field Saturday night to hear presidential candidate and Vermont Senator Bernie Sanders speak, in what is being reported as one of the biggest political rallies so far in the 2016 election cycle.

Addressing the crowd at the University of Denver, Sanders said: "What we are doing tonight is we are sending a message to the billionaire class and that is: You can't have it all!"

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"This campaign is not about me," he continued. "It is not about Hillary Clinton or any other candidate. This campaign is about you, your kids and your parents. It is about creating a political movement of millions of people who stand up and loudly proclaim that this nation belongs to all of us and not just a handful of billionaires."

Saturday's crowd is the latest sign that Sanders is proving a real challenge to former Secretary of State Hillary Clinton, whose rally at New York City's Roosevelt Island last week drew an estimated 5,500 people.

"Sanders's audience—in a state not among those with traditional early nominating contests—rivaled the largest drawn by Clinton and the Vermont senator in recent weeks," wrote Washington Post reporter John Wagner. "The extraordinary turnout was the latest evidence that Sanders, 73, has tapped into the economic anxiety of the Democratic electorate."

Sanders has been drawing large crowds, from Vermont to Minneapolis, and numerous polls show that Sanders is gaining on Clinton, including recent surveys of the battleground state of New Hampshire.