Chris Hughes, who helped Mark Zuckerberg create the company that eventually became Facebook Inc., is calling for the social-media giant to be broken up.

In a nearly 6,000-word opinion essay published online Thursday in the New York Times, Mr. Hughes said the Facebook chief executive has gained power that is both “unprecedented and un-American.”

Mr. Hughes said Mr. Zuckerberg’s focus on growth led him “to sacrifice security and civility for clicks,” and that if Facebook remains intact, its monopoly over online social interaction “will become even more entrenched.”

“Facebook accepts that with success comes accountability. But you don’t enforce accountability by calling for the breakup of a successful American company,” spokesman Nick Clegg said in a statement Thursday. “Accountability of tech companies can only be achieved through the painstaking introduction of new rules for the internet,” he said, adding that Mr. Zuckerberg was scheduled to meet with policymakers this week “to further that work.”

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Mr. Hughes’s rebuke follows growing calls by U.S. and European lawmakers for tougher regulation of internet giants in the wake of data and privacy scandals, and as Facebook faces a possible U.S. Federal Trade Commission fine of up to $5 billion over its consumer-privacy violations.

At a hearing in the House of Representatives on Wednesday, all five members of the FTC said Congress should pass a law addressing data privacy. Lawmakers from both parties agreed but appeared divided over how any such legislation would take shape.

Some lawmakers have urged more drastic action. Sen. Elizabeth Warren, the Massachusetts Democrat running for president, has called for the breakup of Facebook, Amazon.com Inc., and Alphabet Inc.’s Google.

Members of Silicon Valley’s elite, including senior executives from Apple Inc., Google and Microsoft Corp., have called for some form of greater oversight of the tech industry. So has Mr. Zuckerberg, writing in a March op-ed published by the Washington Post and Ireland’s Independent that global regulators should take a “more active role” in governing the internet.

Facebook has taken steps recently to address concerns about online discourse, including by redesigning its website and mobile app. The changes place more emphasis on private groups and visual stories, and less on the platform’s News Feed.

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Mr. Hughes has maintained an active presence in politics since exiting Facebook in 2007 to join former President Obama’s first election campaign. He is a co-chairman of the Economic Security Project, which supports the idea of universal basic income, and is a senior adviser at the Roosevelt Institute, a left-leaning public-policy think tank.

In 2012, he purchased a majority stake in the New Republic and spent years trying to turn the magazine into a more digitally driven media company. He ended up selling the property in 2016.

In his essay, Mr. Hughes said he hasn’t seen Mr. Zuckerberg in person in nearly two years. He said his former Harvard classmate is a “good, kind person” whom the U.S. government needs to hold more accountable for the immense power Facebook wields.

“For too long, lawmakers have marveled at Facebook’s explosive growth and overlooked their responsibility to ensure that Americans are protected and markets are competitive,” Mr. Hughes wrote.

Click here for more from The Wall Street Journal, where this story was first published.