In a speech citing landmark Supreme Court cases and historical figures such as Frederick Douglass, Zuckerberg told an audience at Georgetown University that during times of social upheaval, policymakers have instinctively sought to limit the freedom of speech.

"The impulse is to pull back from free expression," he said. But, he continued, "We are at a crossroads. We can either stand for free expression ... or we can decide the cost is simply too great. We must continue to stand for free expression."

Zuckerberg had said in a Facebook post Wednesday that the speech would reflect his lengthy, "unfiltered" views on threats to free speech worldwide and the speech-related problems that the internet has created.

Speaking with a tinge of wonder and amazement in his voice, Zuckerberg offered soaring assessments of social media's promise, citing powerful movements that organized digitally such as #MeToo and Black Lives Matter - the latter of which, he said, first became a hashtag on Facebook itself.

But invoking civil rights icons like Martin Luther King, Jr. and historic cultural uprisings didn't sit well with Zuckerberg's critics in the civil rights community. Alicia Garza, a cofounder of Black Lives Matter, said Thursday that Zuckerberg had co-opted the organization's name to make a point.

"If he wants to use our movement to claim support for Black communities, then Black lives have to matter more than his bottom line," Garza tweeted.

Color of Change, a civil rights advocacy group, said Facebook needs to be held to a higher standard.

"Facebook's ultimate hypocrisy," it said, "is touting the platform's contributions to social justice movements like Black Lives Matter, but simultaneously rejecting any of the rules or regulations that would actually protect Black lives."

Last spring, the largest Black Lives Matter page on Facebook was shut down after CNN revealed that the page was a scam tied to a middle-aged white man in Australia. The page helped raise thousands of dollars for the movement, but at least some of the money was later transferred to Australian bank accounts. Black Lives Matter had contacted Facebook about the page months earlier, suspecting it was a scam, but Facebook did not suspend the page until CNN's reporting.

During the speech, Zuckerberg pushed back on criticism of Facebook's handling of political advertising, saying he briefly considered banning political ads entirely from Facebook.

"From a business perspective," he said, "the controversy certainly is not worth the very small part of our business that they make up." But he ultimately decided against a ban, he said, concluding that political ads "can be an important part of voice" and that even if Facebook eliminated political ads from its platform, other types of ads could come under scrutiny as well, and it would be difficult to draw a line.

"I don't think most people want to live in a world where you can only post things that tech companies judge to be 100 percent true," Zuckerberg said.

Zuckerberg went on to discuss how tech companies - including his - pose a potential threat to freedom of expression, and returned to a familiar argument: That independent bodies should be responsible for developing rules and norms surrounding content.

He then pivoted to argue for his company's forthcoming oversight board, which will give users the ability to appeal Facebook's content decisions to a panel of outside experts.

Zuckerberg also reiterated another claim: That efforts to restrict US tech companies could lead to unintended consequences such as allowing Chinese competitors to dictate the terms of speech and innovation.

"If another nation's platforms set the rules, our nation's discourse could be set by a completely different set of values," he said. "We should be proactive and write policies that help free expression triumph around the world."

The entrepreneur said his concerns about freedom of expression extend beyond the United States, underscoring how Zuckerberg views himself and his company as being at the center of an epic struggle of ideas that has stretched on for centuries.

"Increasingly, we're seeing countries try to impose their speech restrictions beyond their country," Zuckerberg said during a question-and-answer session with students. He slammed a recent ruling by the European Court of Justice holding that Facebook can be required to delete content worldwide that is considered illegal in individual European countries.

"This is incredibly problematic and troubling," Zuckerberg added, saying there will undoubtedly be future litigation that Facebook will participate in seeking to establish how that ruling should be implemented.

Faced with two questions - one about allegations of Facebook's bias against conservatives and one about Facebook's outreach to conservatives - Zuckerberg quipped, to laughter: "I think it would be hard to be biased against both sides."

Facebook has recently found itself at the center of a debate about truthfulness in political ads after Facebook declined to remove an ad by President Trump's campaign that contained unfounded allegations against former Vice President Joe Biden. The result has been an uproar over Facebook's policy allowing politicians to lie in their campaign advertisements.

Bill Russo, a spokesman for Biden's campaign, accused Zuckerberg on Thursday of having "attempted to use the Constitution as a shield for his company's bottom line."

"His choice to cloak Facebook's policy in a feigned concern for free expression," he added, "demonstrates how unprepared his company is for this unique moment in our history and how little it has learned over the past few years."

Facebook and many legal experts have defended the company's right not to censor political candidates, but some critics including Sen. Elizabeth Warren have slammed the company for running what Warren termed a "disinformation-for-profit machine."

Asked about calls by politicians including Warren for the breakup of Facebook, Zuckerberg referred to a previously leaked recording of his remarks at a companywide meeting at which he'd acknowledged that a Warren presidency could mean an antitrust lawsuit filed by the US seeking that breakup. Zuckerberg at first said he didn't have much more to add. But in the event of a lawsuit to break up the company, he said, "of course we'll stand up for ourselves and for what we believe is right."

Despite the companies' efforts, concerns about the tech industry's handling of offensive speech and disinformation have continued to mount as the 2020 election cycle gets underway.

Facebook has launched a civil rights task force to monitor for election and census interference, and Google (GOOG) has hired some 10,000 employees worldwide to tackle content moderation.

But skeptics say the companies have dragged their feet for too long. At a congressional hearing Wednesday, Oregon Republican Rep. Greg Walden said social media companies have been too "slow to clean up" their platforms while too quick to claim legal immunity for content under a law known as Section 230 of the Communications Decency Act.

"Many of our concerns can be readily addressed if companies just enforce their terms of service," said Walden.