The crowd packed out the California city’s Frank Ogawa Plaza to watch the Democratic senator launch a bid to oust Donald Trump as president in 2020.
“We are here because the American Dream and our American democracy are under attack and on the line like never before,” Ms Harris told supporters. “And we are here at this moment in time because we must answer a fundamental question: Who are we? Who are we as Americans?
“So, let’s answer that question to the world and each other right here and right now. America, we are better than this.”
The first-term US senator announced her candidacy last Monday and launched her formal campaign on Sunday in the city where she served as a prosecutor before becoming California’s attorney general.
If she were to be elected, her victory would be historic: Ms Harris would be the first woman, first female African-American, and first person of Asian descent to become American president.
Addressing supporters in Oakland, Ms Harris invoked the speech that Robert F Kennedy gave in 1968 when he announced that he would challenge president Lyndon B Johnson. She noted Kennedy having said: ”At stake is not simply the leadership of our party and even our country, it is our right to moral leadership of this planet.”
Ms Harris added: “So today I say to you, my friends: These are not ordinary times, and this will not be an ordinary election, but this is our America.”
— Ian Sams (@IanSams)January 27, 2019
The daughter of Jamaican and Indian immigrants, Ms Harris said she had grown up in an East Bay community “with a deep belief in the promise of our country, and a deep understanding of the parts of that promise that still remain unfulfilled”.
Ms Harris's campaign is expected to highlight her career as a prosecutor. She was the first woman elected district attorney in California, as well as the first black woman and first Asian-American to hold that job.
On Sunday, she said she has "only had one client - the people", echoing words she has used in courtrooms and has adopted as her campaign's slogan.
She attacked Mr Trump directly, saying the US welcomed refugees and calling the wall the president wants to build along the US-Mexico border a "medieval vanity project".
Ms Harris is among the first major Democrats to jump into what is expected to be a crowded 2020 presidential contest.
Senator Elizabeth Warren of Massachusetts and Kirsten Gillibrand of New York have announced exploratory committees ahead of 2020 bids.
Former Maryland Republican John Delaney and Julian Castro, federal housing chief under president Barack Obama and a former San Antonio mayor, are already in the race.