"I bet that we will have a legal challenge, and I would bet that we will win the legal challenge," Zuckerberg said in the audio published by The Verge.
Zuckerberg said he didn't want to go against the federal government, but he has to protect his company.
"Does that still suck for us? Yeah. I mean, I don’t want to have a major lawsuit against our own government ... We care about our country and want to work with our government and do good things," Zuckerberg told employees at a July Q&A session at company headquarters. "But look, at the end of the day, if someone’s going to try to threaten something that existential, you go to the mat and you fight."
Zuckerberg later confirmed the audio was him in a Facebook post, writing that "even though it was meant to be internal rather than public, now that it's out there, you can check it out if you're interested in seeing an unfiltered version of what I'm thinking and telling employees on a bunch of topics like social responsibility, breaking up tech companies, Libra, neural computing interfaces, and doing the right thing over the long term."
Warren responded to Zuckerberg's comments Tuesday on Twitter, writing, "What would really 'suck' is if we don’t fix a corrupt system that lets giant companies like Facebook engage in illegal anticompetitive practices, stomp on consumer privacy rights, and repeatedly fumble their responsibility to protect our democracy."
A few hours later, she spoke out again -- though she didn't address Zuckerberg's threat of initiating a lawsuit or whether his doing so could neuter her plan.
"Let's talk a bit about my plan to #BreakUpBigTech and why it's got Mark Zuckerberg so worked up," Warren tweeted, taking Zuckerberg's comments as an opportunity to chart out some of her plan's details.
"My plan to #BreakUpBigTech would undo their illegal, anticompetitive mergers. You'll still be able to use Facebook and Instagram to catch up with friends and family and share photos of your dog. But they'll have to compete with each other to make a better product for you," Warren wrote on Twitter.
She also seemed to address Zuckerberg's criticism in the leaked audio that breaking companies up wouldn't "solve the issues" because "companies can't coordinate and work together" on issues of privacy, foreign interference or hate speech.
"Imagine Facebook and Instagram trying to outdo each other to protect your privacy and keep misinformation out of your feed, instead of working together to sell your data, inundate you with misinformation, and undermine our election security. That's why we need to #BreakUpBigTech," Warren said.
Warren's plan to break up big tech companies targets Facebook for its hold on the tech market, which she thinks makes the company too powerful.
The plan, which Warren announced in March, would "unwind tech mergers that illegally undermine competition," such as Facebook's purchase of WhatsApp and Instagram, Amazon's purchase of Whole Foods and Google's acquisition of the mapping app Waze. The policy, as Zuckerberg said, could be subject to lawsuits.
Zuckerberg's comments in the leaked audio were from two meetings he held with the company addressing impending challenges like user privacy concerns and Warren's crusade to significantly alter the company.
His response was to a question about how "personally worried" he was about "regulators coming in and breaking up Facebook" given Warren's rise in the 2020 presidential race, coupled with the $5 billion fine from the Federal Trade Commission the company recently paid over privacy issues.
This is not the first time Warren has sparred with tech companies over her proposals, often responding more strongly to them than her Democratic opponents.
In late April, Warren got into a back-and-forth dispute with Amazon over her description of the company as a giant corporation using its influence and classified information on its buyers and sellers to create and promote its own products over the competition.
"You can be an umpire, or you can be a player — but you can’t be both," she said, tweeting out her argument to the company point-by-point.
Amazon responded, in part, that sellers aren’t being “knocked out” on their marketplace, but rather, are "seeing record sales every year."
ABC News' Soorin Kim contributed to this report.