The killing of George Floyd has brought an intense moment of racial reckoning in the United States. As protests spread across the country, they have been accompanied by open letters calling for — and promising — change at white-dominated institutions across the arts and academia.

But on Tuesday, a different type of letter appeared online. Titled “A Letter on Justice and Open Debate,” and signed by 153 prominent artists and intellectuals, it began with an acknowledgment of “powerful protests for racial and social justice” before pivoting to a warning against an “intolerant climate” engulfing the culture.

“The free exchange of information and ideas, the lifeblood of a liberal society, is daily becoming more constricted,” the letter declared, citing “an intolerance of opposing views, a vogue for public shaming and ostracism and the tendency to dissolve complex policy issues in a blinding moral certainty.”

“We refuse any false choice between justice and freedom, which cannot exist without each other,” it continues. “As writers we need a culture that leaves us room for experimentation, risk taking, and even mistakes.”

The letter, which was published by Harper’s Magazine and will also appear in several leading international publications, surfaces a debate that has been going on privately in newsrooms, universities and publishing houses that have been navigating demands for diversity and inclusion, while also asking which demands — and the social media dynamics that propel them — go too far.

And on social media, the reaction was swift, with some heaping ridicule on the letter’s signatories — who include cultural luminaries like Margaret Atwood, Bill T. Jones and Wynton Marsalis, along with journalists and academics — for thin-skinnedness, privilege and, as one person put it, fear of loss of “relevance.”

“Okay, I did not sign THE LETTER when I was asked 9 days ago,” Richard Kim, the enterprise director of HuffPost, said on Twitter, “because I could see in 90 seconds that it was fatuous, self-important drivel that would only troll the people it allegedly was trying to reach — and I said as much.”

The debate over diversity, free expression and the limits of acceptable opinion is a long-burning one. But the letter, which was spearheaded by the writer Thomas Chatterton Williams, began taking shape about a month ago, as part of a long-running conversation about these issues with a small group of writers including the historian David Greenberg, the writer Mark Lilla and the journalists Robert Worth and George Packer.

Mr. Williams, a columnist for Harper’s and contributing writer for The New York Times Magazine, said that initially, there was concern over timing.

“We didn’t want to be seen as reacting to the protests we believe are in response to egregious abuses by the police,” he said. “But for some time, there’s been a mood all of us have been quite concerned with.”

He said there wasn’t one particular incident that provoked the letter. But he did cite several recent ones, including the resignation of more than half the board of the National Book Critics Circle over its statement supporting Black Lives Matter, a similar blowup at the Poetry Foundation, and the case of David Shor, a data analyst at a consulting firm who was fired after he tweeted about academic research linking looting and vandalism by protesters to Richard Nixon’s 1968 electoral victory.

Such incidents, Mr. Williams said, both fueled and echoed what he called the far greater and more dangerous “illiberalism” of President Trump.

“Donald Trump is the Canceler in Chief,” he said. “But the correction of Trump’s abuses cannot become an overcorrection that stifles the principles we believe in.”

Mr. Williams said the letter was very much a crowdsourced effort, with about 20 people contributing language. Then it was circulated more broadly for signatures, in what he describes as a process that was both “organic” and aimed at getting a group that was maximally diverse politically, racially and otherwise.

“We’re not just a bunch of old white guys sitting around writing this letter,” Mr. Williams, who is African-American, said. “It includes plenty of Black thinkers, Muslim thinkers, Jewish thinkers, people who are trans and gay, old and young, right wing and left wing.”

“We believe these are values that are widespread and shared, and we wanted the list to reflect that,” he said.

Signatories include the leftist Noam Chomsky and the neoconservative Francis Fukuyama. There are also figures associated with the traditional defense of free speech, including Nadine Strossen, former president of the American Civil Liberties Union, as well as some outspoken critics of political correctness on campuses, including the linguist Steven Pinker and the psychologist Jonathan Haidt.

The signers also include some figures who have lost positions amid controversies, including Ian Buruma, the former editor of the New York Review of Books, and Ronald S. Sullivan Jr., a Harvard Law School professor who left his position as faculty dean of an undergraduate residence amid protests over his legal defense of Harvey Weinstein.

There are also some leading Black intellectuals, including the historian Nell Irvin Painter, the poets Reginald Dwayne Betts and Gregory Pardlo, and the linguist John McWhorter. And there are a number of journalists, including several opinion columnists for The New York Times.

Nicholas Lemann, a staff writer for The New Yorker and a former dean of Columbia Journalism School, said that he rarely signs letters, but thought this one was important.

“What concerns me is a sense that a lot of people out there seem to think open argument over everything is an unhealthy thing,” he said. “I’ve spent my whole life having vigorous arguments with people I disagree with, and don’t want to think we are moving out of this world.”

The principle of open argument, he added, becomes especially important outside liberal-leaning enclaves, “where people don’t have the option of shutting down these supposedly completely unacceptable views.”

Mr. Pardlo said that as somebody who has felt the “chilling effect” of being the only person of color in predominantly white institutions, he hoped the letter would spark conversation about those “chilling forces, no matter where they come from.”

He said he was surprised by some of the blowback to the letter.

“It seems some of the conversation has turned to who the signatories are more than the content of the letter,” he said.

There was particularly strong blowback over the inclusion of J.K. Rowling, who has come under fierce criticism over a series of comments widely seen as anti-transgender.

Emily VanDerWerff, a critic at large at Vox who is transgender, posted on Twitter a letter she said she had sent to her editors, criticizing the fact that the Vox writer Matthew Yglesias had signed the letter, which she said was also signed by “several prominent anti-trans voices” — but noted that she was not calling for Mr. Yglesias to be fired or reprimanded.

Doing so “would only solidify, in his own mind, the belief that he is being martyred,” she wrote.

Mr. Yglesias declined to comment except to say that he has long admired Ms. VanDerWerff’s work and continued to “respect her enormously.”

Amid the intense criticism, some signatories appeared to back away from the letter. On Tuesday evening, the historian Kerri K. Greenidge tweeted “I do not endorse this @Harpers letter,” and said she was in touch with the magazine about a retraction. (Giulia Melucci, a spokeswoman for Harper’s, said the magazine had fact-checked all signatures and that Dr. Greenidge had signed off. But she said the magazine is “respectfully removing her name.”)

Another person who signed, who spoke on the condition of anonymity in an effort to stay out of the growing storm, said she did not know who all the other signatories were when she agreed to participate, and if she had, she may not have signed. She also said that the letter, which was about internet shaming, among other things, was now being used to shame people on the internet.

But Mr. Betts, the director of the Million Books Project, a new effort aimed at getting book collections to more than 1,000 prisons, was unfazed by the variety of signers.

“I’m rolling with people I wouldn’t normally be in a room with,” he said. “But you need to concede that what’s in the letter is worthy of some thought.”

He said that as someone who had spent more than eight years in prison for a carjacking committed when he was a teenager, he was given pause by what he called the unforgiving nature of the current moment. “It’s antithetical to my notion of how we need to deal with problems in society,” he said.

He cited in particular the case of James Bennet, who resigned as the editorial page editor of The New York Times following an outcry over an Op-Ed by Senator Tom Cotton, Republican of Arkansas, and cases of authors of young adult literature withdrawing books in the face of criticism over cultural appropriation.

“You can criticize what people say, you can argue about platforms,” Mr. Betts said. “But it seems like some of the excesses of the moment are leading people to be silenced in a new way.”

Eileen Murphy, a Times spokeswoman, declined to comment.

Mr. Williams said he was trying to think through how outrage over Mr. Floyd’s death had become so intertwined with calls for change “at organizations that don’t have much to do with the situation George Floyd found himself in.”

But to him, he said, the cause of the letter is clear: “It’s a defense of people being able to speak and think freely without fear of punishment or retribution, of the right to disagree and not fear for your employment.”