• Ms Harris challenged Mr Biden to disavow his past opposition to bussing in America.
• The candidates are vying to take on Donald Trump in next year's election.
Democratic White House front-runner Joe Biden has come under fierce attack for his record on race in a televised debate with nine rivals.
Senator Kamala Harris assailed him for touting his past work with bigoted senators and having once opposed a policy to foster diversity in schools.
He said she had "mischaracterised" his position, insisting he had entered politics to champion civil rights.
The candidates are vying to take on Donald Trump in next year's election.
How did the flashpoint occur?
The clash of the night unfolded midway through Thursday night's forum in Miami, Florida.
Ms Harris - the only black woman in the Democratic field of 20 candidates - pilloried Mr Biden for this month having cited his work decades ago in the Senate with fellow Democrats who had favoured segregation of the races.
Turning to him, she said: "I do not believe you are a racist and I agree with you when you commit yourself to the importance of finding common ground.
"But I also believe, and it's personal and I was actually very, it was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country."
She also took him to task for his political role in the mid-1970s fighting against sending white children to majority-black schools in other neighbourhoods, and vice versa.
"And it was not only that," the 54-year-old said, "but you also worked with them [racist senators] to oppose bussing.
"And there was a little girl in California who was part of the second class to integrate her public school and she was bussed to school every day. And that little girl was me."
Mr Biden - who said last week he "detested" what the segregationists stood for - bristled: "It's a mischaracterisation of my position across the board.
"I did not praise racists. That is not true."
Ms Harris challenged Mr Biden to disavow his past opposition to bussing in America.
He insisted he was only against the policy being mandated by the federal government, but had no problem with its implementation at a state level.
Mr Biden also cited his tenure serving two terms as vice-president to Barack Obama, America's first black president.
Who else was on stage?
Debate-watchers had been expecting a duel between Mr Biden and Bernie Sanders.
Mr Biden, a pragmatic centrist, and Mr Sanders, a Democratic socialist, have been battling for the soul of the party as it ponders how best to take on President Trump.
Mr Sanders, a Vermont senator, did dominate the first half hour by defending his plans for free healthcare for all and denouncing Mr Trump as "a pathological liar and a racist".
Also in the fray was another top-tier contender, Pete Buttigieg, the 37-year-old gay mayor of South Bend, Indiana.
The other six on stage have all been polling at one per cent or less: Senators Kirsten Gillibrand and Michael Bennet, former Colorado Governor John Hickenlooper, congressman Eric Swalwell, self-help guru Marianne Williamson and entrepreneur Andrew Yang.
Where did it all go wrong for Biden?
Ms Harris' ambush was not the first time in the debate before a national TV audience of millions that Mr Biden came under attack.
The 76-year-old found himself on the defensive early on an issue that he has presented as one of his strengths - political longevity.
Mr Swalwell said: "I was six years old when a presidential candidate came to the California Democratic convention and said, 'It's time to pass the torch to a new generation of Americans.'
"That candidate was then-Senator Joe Biden. Joe Biden was right when he said it was time to pass the torch to a new generation Americans 32 years ago - he's still right today."
Mr Biden, who would be the oldest president ever elected, retorted: "I'm still holding on to that torch."
The former Delaware senator's debate game plan had been to consolidate his status as the front-runner after recent missteps.
He has flip-flopped on abortion and recanted after provoking liberal ire for calling Vice-President Mike Pence "a decent guy".
But as pack leader, he found himself with a big bullseye on his back.
Where did it all go right for Harris?
Until now, Ms Harris has failed to match the heavyweights' star power in a crowded Democratic field, though she has remained one of the few top-tier candidates since launching her campaign in January.
She soon grabbed the limelight on Thursday night. The mauling of Mr Biden was not her only standout moment.
As the debated unravelled at one point into a free-for-all shouting match, she was cheered for saying: "America does not want to witness a food fight - they want to know how we're going to put food on their table!"
Railing against President Trump at another point, she said with an emphasis on the female pronoun: "I will ensure that this microphone - that the president of the United States holds in her hand - is used in a way that is about reflecting the values of our country."
Ms Harris - who is the daughter of Jamaican and Indian immigrants - is not without her own political baggage, however.
On stage, Mr Biden pointed out he had been a criminal defence lawyer, in a veiled jab at her prior career as a public prosecutor.
She has been forced to defend her record as a San Francisco district attorney, amid claims she breached the rights of defendants and opposed criminal justice reforms.