“People having the power to express themselves at scale is a new kind of force in the world — a Fifth Estate alongside the other power structures of society,” Mr. Zuckerberg, 35, said.

He added that despite the messiness of free speech, “the long journey towards greater progress requires confronting ideas that challenge us.”

“I’m here today because I believe we must continue to stand for free expression,” he said.

The address was an unusually public doubling down by the tech billionaire on a free speech stance that has been highly criticized. It was a sign of how Mr. Zuckerberg was trying to reposition Facebook in a politicized environment where the company had been accused of amplifying disinformation, hate speech and violent content. Facebook is also under scrutiny for the power it wields over social media and is grappling with emboldened governments around the world that want to regulate it.

In recent weeks, Facebook’s challenges have piled up. In Europe, the region’s top court ruled this month that individual countries could order the company to take down posts not only in their own countries but elsewhere. Several of Facebook’s partners on a cryptocurrency initiative dropped out after regulators complained. And the company found itself in the eye of a storm in the United States — including with Ms. Warren, the Massachusetts senator who is running for the Democratic presidential nomination — over how to treat political speech.

But even as Mr. Zuckerberg tried to strike back on Thursday, his speech was blasted.

“Zuckerberg attempted to use the Constitution as a shield for his company’s bottom line, and his choice to cloak Facebook’s policy in a feigned concern for free expression demonstrates how unprepared his company is for this unique moment in our history and how little it has learned over the past few years,” said Bill Russo, a spokesman for the presidential campaign of former Vice President Joseph R. Biden Jr.