By Deepa Seetharaman and Jeff Horwitz

Facebook Inc. Chief Executive Mark Zuckerberg, addressing employees in a highly charged town-hall meeting on Tuesday, defended his decision to preserve a controversial post from President Trump but said he was open to some tweaks in how the company deals with such content.

Mr. Zuckerberg spoke amid mounting outrage from insiders and civil-rights activists that the message -- in which Mr. Trump called protesters thugs and warned: "When the looting starts, the shooting starts" -- was tantamount to a call for violence, and therefore a violation of Facebook's rules. Facebook took no action on the post, while Twitter Inc. shielded the same message from public view along with a label saying it glorified violence.

Mr. Zuckerberg disagreed that Mr. Trump's post broke Facebook's rules, but he said he would dispatch teams to study other options for handling objectionable posts beyond taking them down or leaving them up, according to a person who attended the meeting. Mr. Zuckerberg said he would personally review the options submitted by the teams.

The employee meeting, which was initially scheduled for Thursday and was moved earlier, came a day after some employees participated in a "virtual walkout" opposing the policy decision, with more than a dozen airing their grievances on Twitter. Two software engineers publicly said they quit the company, in part because of what they called Facebook's failure to enforce its own rules when it comes to Mr. Trump.

The meeting wasn't streamed publicly, as some internal Facebook events have been recently, but employees said Mr. Zuckerberg didn't give ground on the issue.

Few employees expected Mr. Zuckerberg to change his stance going into Tuesday's town hall, according to people who attended the meeting. Some internal comments on Mr. Zuckerberg's livestream reflected a "deep sadness" about his stance, one of the people said.

One employee asked who Mr. Zuckerberg had talked to when deciding how to address Mr. Trump's posts. The only black person Mr. Zuckerberg mentioned was the company's head of diversity, according to an employee. Mr. Zuckerberg also said the company consulted with outside groups before choosing its current course, but didn't specify which ones.

Mr. Zuckerberg also argued that employees should view the controversy within the larger context of activity on the platform. The initial posting on Facebook of the video of George Floyd's arrest and death while in police custody in Minneapolis, a post that led to widespread outrage about those events, showed that the platform was a force for good, he said. Mr. Zuckerberg said that should be weighed against concerns about how the president was using the platform.

Mr. Zuckerberg also noted his private philanthropic work related to injustice in criminal sentencing and criticized other companies' statements of support for Black Lives Matter.

"We're kind of seeing every corporate CEO across the country right now just stand up and say, 'All right, yeah, black lives matter, we stand with our black community,' " Mr. Zuckerberg said, according to a person who attended the meeting. "I think that's important to say and to remind people to say it, but I don't think it takes any particular courage to say those things when there's a huge crisis. What I hope people can look at is the track record that I and other leaders have of focusing on these issues."

Mr. Zuckerberg has some support among Facebook's more than 48,000 full-time employees, according to people inside the company, and many employees have elected not to wade into the tense political debate. A vocal faction of the company has indicated they aren't satisfied with the company's stance.

"It's crystal clear today that leadership refuses to stand with us," said Brandon Dail, a Facebook engineer, in a Twitter post.

Mr. Zuckerberg has said he doesn't believe private companies should regulate political speech and that while he personally found Mr. Trump's posts "deeply offensive," he thinks it is better for the debate over his comments to be held publicly rather than suppressed. Employees have pushed back, in some cases publicly, saying Facebook needs to enforce its content rules, rather than make exceptions for powerful political leaders. Some have also said the company should take more responsibility for the way its platform allows incendiary content to spread faster than more moderate views.

"Open and honest discussion has always been a part of Facebook's culture, " Facebook said in a written statement. "Mark had an open discussion with employees today, as he has regularly over the years. He's grateful for their feedback."

Facebook has weathered multiple crises in recent years, especially since the 2016 election, but the current employee turmoil amounts to one of the toughest challenges to Mr. Zuckerberg's leadership since he co-founded the company 16 years ago.

Since late last week, Mr. Zuckerberg and Chief Operating Officer Sheryl Sandberg have discussed the policy decision with employees, especially black executives and employees, as well as civil-rights leaders. Some participants said they found those meetings largely unsatisfying.

Three civil-rights leaders -- Vanita Gupta, president and CEO of the Leadership Conference on Civil and Human Rights, Sherrilyn Ifill, president and director-counsel of the NAACP Legal Defense and Educational Fund Inc. and Rashad Robinson, president of Color of Change -- issued a fiery statement after speaking with Mr. Zuckerberg and Ms. Sandberg late Monday.

"We are disappointed and stunned by Mark's incomprehensible explanations for allowing the Trump posts to remain up," they said. "He did not demonstrate understanding of historic or modern-day voter suppression and he refuses to acknowledge how Facebook is facilitating Trump's call for violence against protesters."

Timothy Aveni, an engineer who quit Facebook this week over the policies, said he was disillusioned because he felt Mr. Zuckerberg wasn't enforcing his own rules. "Mark always told us that he would draw the line at speech that calls for violence," Mr. Aveni wrote on Facebook. "He showed us on Friday that this was a lie."

Mr. Zuckerberg ended the 90-minute session by affirming his belief that Facebook was ultimately a force for good in the world. "The net impact of the different things we're doing in the world is positive," he told employees, according to a person familiar with his remarks. "I really believe it is."

Write to Deepa Seetharaman at Deepa.Seetharaman@wsj.com and Jeff Horwitz at Jeff.Horwitz@wsj.com