SAN FRANCISCO — On Twitter, users called on Wednesday for the company’s chief executive, Jack Dorsey, to take down President Trump’s account.
Civil rights groups weighed in, saying action by social media companies against calls for political violence was “long overdue.” And even venture capitalists who had reaped riches from investing in social media urged Twitter and Facebook to do more.
“For four years you’ve rationalized this terror. Inciting violent treason is not a free speech exercise,” Chris Sacca, a tech investor who had invested in Twitter, wrote to Mr. Dorsey and Facebook’s chief executive, Mark Zuckerberg. “If you work at those companies, it’s on you too. Shut it down.”
As pro-Trump protesters stormed the Capitol building on Wednesday and halted the certification of Electoral College votes, the role of social media companies such as Twitter, Facebook and YouTube in spreading misinformation and being a megaphone for Mr. Trump came under renewed criticism.
For years, Mr. Trump had built his influence with rapid-fire tweets and by reaching out to millions of people on Facebook. Since losing November’s election, he had used the platforms to challenge the election results and call them fraudulent.
Twitter, Facebook and others had long resisted cracking down on Mr. Trump’s posts and other toxic content. While the platforms had started taking more steps against political misinformation in the months before the election, they declined to remove Mr. Trump’s posts and instead took half steps, such as labeling his posts.
So when violence broke out in Washington on Wednesday, it was, in the minds of longtime critics, the day the chickens came home to roost for the social media companies.
“We know the social media companies have been lackadaisical at best” at stopping extremism from growing on their platforms, said Jonathan Greenblatt, director of the Anti-Defamation League. “Freedom of expression is not the freedom to incite violence. That is not protected speech.”
Renee DiResta, a researcher at the Stanford Internet Observatory who studies online movements, added that the violence was the result of people operating in closed social media networks where they believed the claims of voter fraud and of the election being stolen from Mr. Trump.
“This is a demonstration of the very real-world impact of echo chambers,” she said. “This has been a striking repudiation of the idea that there is an online and an offline world, and that what is said online is in some way kept online. I hope that this eliminates the conception from people’s minds.”
Facebook, Twitter and YouTube said on Wednesday that they were reviewing the situation and would not tolerate calls for violence on their sites. In a statement, Twitter said it would take action against tweets that violated its policies and was “exploring other escalated enforcement actions.”
YouTube said it removed multiple live streams that showed participants storming the Capitol building carrying firearms. It also said it would elevate authoritative news sources on its home page, search results and in recommendations.
“The violent protests in the Capitol today are a disgrace,” added Andy Stone, a Facebook spokesman. “We prohibit incitement and calls for violence on our platform. We are actively reviewing and removing any content that breaks these rules.”
Mr. Trump also told his supporters to go home in a video that he posted on multiple social media sites on Wednesday afternoon. “You have to go home now. We have to have peace. We have to have law and order,” he said, while repeating false claims that the election had been stolen from him.
Twitter later added a label to Mr. Trump’s video, saying its claims of fraud were disputed and could lead to violence. Facebook and YouTube removed the video entirely. Guy Rosen, a Facebook executive, said the social network took down the video because it was “an emergency situation” and the video “contributes to rather than diminishes the risk of ongoing violence.”
Critics said the statements by Facebook, Twitter and YouTube were too little, too late, after calls for violence and plans for protests had already spread on the platforms.
On Facebook, protesters had openly discussed what they aimed to do in Washington on a Facebook page called Red-State Secession for weeks. The page had asked its roughly 8,000 followers to share addresses of perceived “enemies” in the nation’s capital, including the home addresses of federal judges, members of Congress and prominent progressive politicians.
Comments left on the page often featured photos of gun and ammunition, along with emojis suggesting that members of the group were planning for violence. One post on Tuesday said people should be “prepared to use force to defend civilization.” Several comments below the post showed photos of assault rifles, ammunition and other weapons. In the comments, people referred to “occupying” the capital, and taking action to force Congress to overturn the results of the elections.
Facebook said it removed Red-State Secession on Wednesday morning. Before it was taken down, the page directed followers to other social media sites like Gab and Parler that have gained popularity in right-wing circles since the election.
Those alternative social media sites were rife with Trump supporters organizing and communicating on Wednesday. On Parler, one trending hashtag was #stormthecapitol. Many Trump supporters on the sites also appeared to believe a false rumor that Antifa, a left-wing movement, was responsible for committing violence at the protests.
“WAKE UP AMERICA, IT’S ANTIFA and BLM operatives who are committing the violence, NOT TRUMP SUPPORTERS!,” said one Parler account member called @Trumpfans100, offering no evidence for the claims.
Officials at Parler did not immediately respond to a request for comment.
What Facebook, Twitter and YouTube may do next is unclear. Over the past year, some of the companies have stepped up efforts to moderate Mr. Trump’s account, though they have stopped short of taking down his posts. Twitter began adding labels to Mr. Trump’s false and misleading tweets last year and has prevented users from sharing the posts to limit their spread. Facebook has also attached labels to some of Mr. Trump’s posts, redirecting users to reliable and accurate data.
At both Facebook and Twitter, executives met on Wednesday to figure out how to react, according to people with knowledge of the companies. Facebook employees have combed the social network for public examples of protest organization and have taken down a number of hashtag terms that were used to coordinate the protests on Wednesday.
When Mr. Trump leaves office, the companies may have a freer hand. On Twitter, Mr. Trump has enjoyed exceptions to its rules because the company has said it considers posts from world leaders to be in the public interest. But Twitter has said that after Mr. Trump is no longer president, he will be treated like a regular user and have his tweets subject to deletion.
Before then, the pressure remains on the tech companies.
“This level of insurrection should not exist, whether it is on the Twitter platform from the president, or whether it’s on Facebook, which allows people to recruit and carry out these types of dangerous activities,” Derrick Johnson, president and chief executive of the N.A.A.C.P., said in an interview. “This is an example of where both our democracy is being undermined and where people are not being kept safe. They must suspend the president’s account immediately.
Daisuke Wakabayashi contributed reporting.