SAN FRANCISCO — Facebook on Thursday said it will block President Trump on its platforms at least until the end of his term on Jan. 20, as the mainstream online world moved forcefully to limit the president after years of inaction.
Mark Zuckerberg, Facebook’s chief executive, said in a post that the social network decided to cut off Mr. Trump because a rampage by pro-Trump supporters in the nation’s capital a day earlier, which was urged on by the president, showed that Mr. Trump “intends to use his remaining time in office to undermine the peaceful and lawful transition of power to his elected successor, Joe Biden.”
“We believe the risks of allowing the president to continue to use our service during this period are simply too great,” Mr. Zuckerberg wrote. As a result, he said, Facebook and its photo-sharing site Instagram would extend blocks, first put in place on Wednesday, on Mr. Trump’s ability to post “until the peaceful transition of power is complete.”
The move was part of a widening revolt by social media companies against Mr. Trump, who has used the sites throughout his presidency to rile up his supporters and bully his enemies.
Twitter on Wednesday said it would lock Mr. Trump’s account for 12 hours because he had posted several tweets that violated its rules against calling for violence and discrediting the vote. Snap, the maker of Snapchat, also cut off access to Mr. Trump’s Snapchat account. And YouTube on Thursday implemented a stricter election fraud misinformation policy to make it easier to suspend the president for posting false election claims.
The actions were a striking change for a social media industry that has long declined to take down Mr. Trump’s posts, which were often filled with falsehoods and threats. Facebook and Twitter positioned themselves as defenders of free speech and public discussion, saying it was in people’s interests to see what world leaders posted, even as critics assailed them for allowing misinformation and toxic content to flow unimpeded.
Lawmakers and even employees of the companies said the platforms had waited too long to take serious action against Mr. Trump. At Facebook, dozens of employees noted that the company had only suspended Mr. Trump after Democrats had secured the presidency and control of the Senate, according to people familiar with the internal conversations.
“While I’m pleased to see social media platforms like Facebook, Twitter and YouTube take long-belated steps to address the president’s sustained misuse of their platforms to sow discord and violence, these isolated actions are both too late and not nearly enough,” said Senator Mark Warner, Democrat of Virginia.
The spotlight now falls on Twitter and what it will do with Mr. Trump’s account. The social media service has been Mr. Trump’s preferred megaphone, where he has more than 88 million followers, compared with 35 million on Facebook. Twitter’s locking of Mr. Trump’s account on Wednesday was set to be lifted on Thursday if he complied with the service’s demand to delete several tweets.
Twitter said in a statement that it was continuing to evaluate the situation and whether “further escalation in our enforcement approach is necessary.” On Wednesday, the company had said the risks of keeping Mr. Trump’s commentary live on its site had become too high.
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Derrick Johnson, the president and chief executive of the NAACP, praised Facebook’s decision to lock Mr. Trump’s account, and said he urged Twitter to do the same.
“The president’s social media accounts are a petri dish of disinformation, designed to divide and fuel violence at all costs,” Mr. Johnson said.
A spokesman for the White House did not immediately respond to a request for comment.
In recent months, Twitter and Facebook had begun to push back on the president’s posts, adding fact-checking labels to some of his most incendiary statements. Mr. Trump fired back, signing an executive order intended to strip legal protections from the social media companies and claiming they were censoring conservative voices.
At Facebook, Mr. Zuckerberg and other executives had given Mr. Trump significant leeway on his Facebook account, often allowing the president’s false statements to stay up on the network despite heavy criticism.
Mr. Zuckerberg repeatedly said he did not want Facebook to be “the arbiter of truth” in political discourse and that he believed strongly in protecting speech across Facebook, the platform he founded that is now used by more than three billion people globally.
“We did this because we believe that the public has a right to the broadest possible access to political speech, even controversial speech,” Mr. Zuckerberg said in his post on Thursday.
“The current context is now fundamentally different, involving use of our platform to incite violent insurrection against a democratically elected government,” Mr. Zuckerberg said.
Daisuke Wakabayashi and Sheera Frenkel contributed reporting.