Hillary Rodham Clinton, in a speech that was at times sweeping and at times policy laden, delivered on Saturday a pointed repudiation of Republican economic policies and a populist promise to reverse the gaping gulf between the rich and poor at her biggest campaign event to date.

Under sunny skies and surrounded by flag-waving supporters on Roosevelt Island in New York, Mrs. Clinton pledged to run an inclusive campaign and to create a more inclusive economy, saying that even the new voices in the Republican Party continued to push “the top-down economic policies that failed us before.”

“These Republicans trip over themselves promising lower taxes for the wealthy and fewer rules for the biggest corporations without any regard on how that will make income inequality worse,” she said before a crowd estimated at 5,500, according to the campaign.

“I’m not running for some Americans, but for all Americans,” Mrs. Clinton said. “I’m running for all Americans.”