"I'd like to talk about who we're running against," Warren said. "A billionaire who calls women fat broads and horse faced lesbians, and no, I'm not talking about Donald Trump, I'm talking about Mayor Bloomberg.
"Democrats are not going to win if we have a nominee who has a history of hiding his tax returns, of harassing women," she added.
Warren also called out Bloomberg for his support of policies that hurt people of color while he was mayor of New York City.
"And of supporting racist policies like red lining and stop and frisk. Look, I'll support whoever the Democratic nominee is, but understand this. Democrats take a huge risk if we just substitute one arrogant billionaire for another," Warren said.
Klobuchar then followed Warren by saying that she welcomed Bloomberg to the stage because "I thought that he shouldn't be hiding behind his TV ads," before noting that a memo from the Bloomberg campaign this morning seemed to show he wanted women to step aside from the Democratic nomination.
"Because we should pave the way for him to become the nominee," said Klobuchar. "I have been told as a woman — someone no one thought would still be standing up on this stage, but I am because of pure grit and the people out there — I've been told many times to wait my turn and to step aside. And I'm not going to do that now."
Bloomberg initially responded by touting his record in New York, citing it as proof he could beat Trump in an election.
"I am a New Yorker," he said. "I know how to take on an arrogant conman like Donald Trump."
Later in the debate, Bloomberg was specifically asked about the hostile work environment reported by some former female colleagues, some of whom have signed non-disclosure agreements with the company.
"I have no tolerance for the kind of behavior the #MeToo movement has exposed," Bloomberg said.
"Anybody that does anything wrong in our company, we investigate it, and if it's appropriate, they're gone that day," he added, before listing how many women in leadership roles he has hired at his company, during his time as mayor, and at his foundation. He noted that 70% of the foundation employees are women.
But Warren wasn't satisfied by the billionaire's response and called on him to release his former employees from their NDAs.
"I hope you heard his defense: 'I've been nice to some women.' That just doesn't cut it," she said.