By Deepa Seetharaman

Facebook Inc. employees staged a virtual walkout Monday and some publicly denounced CEO Mark Zuckerberg's decision to leave up a post from President Trump about the recent social unrest, comments they believe violated the company's rules about inciting violence.

Over the weekend, more than a dozen employees spoke out on Twitter against Mr. Zuckerberg's decision to keep up a post from the president, which called demonstrators thugs and warned: "When the looting starts, the shooting starts." Hundreds of employees were part of internal groups and chat threads to coordinate the walkout, according to a Facebook employee.

The walkout was confirmed by a company spokeswoman.

Although employee activism has been common around Silicon Valley in recent years, the public outcry is unusual for Facebook employees, who have typically kept their disagreements in-house over the past several years of scandals. But the events of the last few days pushed these debates into public view, mirroring similar developments at rival tech companies like Alphabet Inc. and Amazon.com Inc.

Facebook says it refrains from fact-checking or removing politicians' posts on the platform but will take down posts that glorify violence and spread voter misinformation. Some employees and outside academics who study Facebook's content rules said the looting post, along with an earlier one that contained inaccuracies about voting by mail, broke the company's rules.

"I'm a FB employee that completely disagrees with Mark's decision to do nothing about Trump's recent posts, which clearly incite violence," tweeted Jason Stirman, who lists himself as a design manager at Facebook on his LinkedIn page. "I'm not alone inside of FB. There isn't a neutral position on racism."

On Friday, Mr. Zuckerberg said those posts would remain, despite his own view that the looting post was "deeply offensive." He said that even though he knew many disagreed, he believed it was "better to have this discussion out in the open, especially when the stakes are so high."

Late on Sunday night, Mr. Zuckerberg said Facebook would commit $10 million to groups working on racial justice.

Civil rights leaders blasted Facebook's decision. Rashad Robinson, executive director of Color Of Change, said he was "not only just underwhelmed, but insulted, by the response from Facebook and Mark Zuckerberg in response to the nationwide protests." Mr. Robinson and Vanita Gupta, head of the Leadership Conference on Civil and Human Rights, said they would be speaking to Mr. Zuckerberg and Chief Operating Officer Sheryl Sandberg Monday evening.

Mr. Trump's tweet was in the early stages of the protests sparked by the death last week of George Floyd, a black man, while in police custody in Minneapolis. Those protests have spread nationwide in the days since.

"Mark is wrong, and I will endeavor in the loudest possible way to change his mind," said Ryan Freitas, director of product design on news feed at Facebook, in a tweet.

In a statement, the Facebook spokeswoman said, "We recognize the pain many of our people are feeling right now, especially our Black community. We encourage employees to speak openly when they disagree with leadership. As we face additional difficult decisions around content ahead, we'll continue seeking their honest feedback." The New York Times earlier reported the walkout.

The decisions and policies being protested are "consistent with what the company has been saying forever," said Kate Klonick, an assistant professor at St. John's Law School who has studied the platform's content moderation on the platform. But by last night, employees were so disenchanted that some began changing their internal company profiles to display Twitter's logo. "I'm in awe of how quickly this has snowballed," Ms. Klonick said.

Even though Facebook has taken a hands-off approach to the president, he has continued to include the company in his complaints about unfair treatment for conservatives, a beef that led him last week to issue an executive order that would strip some companies of one of their most important legal protections.

Twitter, by contrast, shielded from public view the looting tweet from Mr. Trump. It can now be seen only after users click a box with a notice saying it violated Twitter's rules against encouraging violence. Another tweet from the president, in which he said voting by mail would lead to rampant fraud, was affixed with a label encouraging users to "Get the facts about mail-in ballots" and referring them to other sources of information.

Mr. Trump and others, largely from the right, have argued that the moves are akin to censorship and that private companies shouldn't be in the business of regulating political speech.

The same messages were cross-posted to Mr. Trump's Facebook page, and no action has been taken against them.

One Facebook employee said part of the issue is that Facebook hasn't given itself the same options as Twitter when dealing with such posts.

"@Facebook's recent decision to not act on posts that incite violence ignores other options to keep our community safe," tweeted Sara Zhang, a Facebook product designer. "The policy pigeon holes us into addressing harmful user-facing content in two ways: keep content up or take it down."

The employee unrest adds to the difficulties confronting Facebook. Last week, The Wall Street Journal reported that Facebook executives largely shelved internal research about the platform's contribution to polarization in part because remedies could have an outsized effect on American conservatives. On Monday, one current employee described those actions by senior executives as " inexcusable."

The angry tone within Facebook is a contrast to the last several weeks, when morale was generally high over the company's response to the new coronavirus. Many employees felt the company had regained purpose and supported Mr. Zuckerberg's steps to elevate accurate information about the virus and rely on world health experts.

Mr. Trump's tweet saying "when the looting starts, the shooting starts" was a line that some people took to refer to the former police chief of Miami when he cracked down on U.S. civil-rights protests, as well as the former governor of Alabama, known for his opposition to the U.S. civil-rights movement.

Mr. Trump later defended his message as being misunderstood. "It was spoken as a fact, not as a statement," he said in a subsequent tweet.

--Jeff Horwitz contributed to this article.

Write to Deepa Seetharaman at Deepa.Seetharaman@wsj.com