Mrs Clinton is outlining her key policies in a speech to thousands of supporters on New York City’s Roosevelt Island.
Her campaign team hopes to boost approval ratings, which have flagged since her soft-launch in April.
Her husband, former President Bill Clinton, and daughter Chelsea are appearing alongside her.
In a video released on Friday, Mrs Clinton, 67, put herself forward as the champion of the “everyday American”.
“Everyone deserves a chance to live up to his or her God-given potential. That’s the dream we share. That’s the fight we must wage,” she said.
“My dad, the son of a factory worker, could start a small business, my mom, who never got to go to college, could see her daughter go to college.”
She added: “Everyday Americans and their families need a champion, a champion who will fight for them every single day and I want to be that champion.”
So far, the former US secretary of state has held small events with selected audiences in early voting states such as Iowa. Saturday’s outdoor rally marks a change in gear for her campaign.
Mrs Clinton hopes to make history as the first female US president. If successful she would also keep the White House within the same party for a third term.
She is not expected to detail specific policy proposals on Saturday. Her aides say that will happen over the next few weeks on issues including the economy, jobs and college affordability.
Jennifer Palmieri, the Clinton campaign’s communications director, said Mrs Clinton plans to give a policy address almost every week in the coming months.