Police estimated the crowd at 100,000. People packed into the block-sized park, into side streets and up the steps of the state Capitol.
"Do you ever have small crowds in Denver? Goodness gracious," Obama said as he took the stage.
Obama accepted the Democratic nomination in Denver two months ago before a crowd of about 85,000.
On Sunday, Obama responded to Republican John McCain's criticism of his economic philosophy. Earlier Sunday on "Meet the Press," McCain said Obama's response to a tax question from an Ohio plumber revealed the Democrat's very liberal ideas.
"We just figured it out with 'Joe the Plumber.' Americans just figured it out. He wants to spread the wealth around," McCain said. "He wants to raise taxes in a time of economic difficulties."
Obama defended his tax plan, saying he would not raise taxes on people who make less than $250,000 a year.
"If you make less than a quarter of a million dollars a year - which includes 98 percent of small business owners and 99.9 percent of all plumbers - you won't see your taxes increase one single dime," Obama said.
Then he repeated one of his campaign's main strategies - trying to tie McCain to President Bush.
"We're not going to let George Bush pass the torch to John McCain. It's time for change. And that's why I'm running for president of the United States," Obama said.
Obama called for a second stimulus plan targeted toward the middle class, including an "American jobs tax credit" for businesses that create new jobs.
He also said he would invest in rural broadband and spend $15 billion a year on renewable energy.
"And if people ask how we're going to pay for this, you tell them that if we can spend $10 billion a month in Iraq, we can spend $10 billion here in America," Obama said.
However, the United States went into debt to pay for the Iraq war, so redirecting war spending to Obama's programs would still add to the deficit.
Obama also made a stop in Fort Collins on Sunday. His wife, Michelle, is scheduled to be in Colorado Springs on Tuesday.
The McCain campaign has not yet announced any Colorado events this week.