Twitter and Facebook moved Wednesday to curb the reach of President Donald Trump's posts questioning the vote-counting process as a battle over the knife-edge US election spilled into social media.
Twitter and Facebook acted after saying the president violated platform rules in claiming ballot irregularities from Tuesday's vote.
Trump alleged that there had been "surprise ballot dumps" in states where he had been leading Democrat Joe Biden in the race for the White House.
Twitter's action made the comments less visible, and users seeking to read the post were required to click through a warning that "some or all of the content shared in this Tweet is disputed and might be misleading."
A Twitter spokesperson said the action was taken "in line with our Civic Integrity Policy," and would "significantly restrict engagements" with the tweet.
A similar action was taken against an earlier tweet by the president suggesting the Democratic nominee was seeking to "steal" the election.
The Twitter spokesperson said it took action on a number of other comments including premature victory claims by a North Carolina Republican Senate candidate and one comment contending prematurely that Biden had won Wisconsin.
"As votes are still being counted across the country, our teams continue to take enforcement action on Tweets that prematurely declare victory or contain misleading information about the election broadly," Twitter said.
Facebook also added disclaimers to messages by Trump as social platforms scrambled to respond.
The leading social network labeled the posts with a disclaimer stating that final results may differ from initial vote counts.
Some Trump critics said the actions were insufficient.
"They absolutely need to take down, not just flag with a weak interstitial (message)," said Jessica Gonzalez of the activist group Free Press.
"Take down disinformation about our democracy. We're on the brink here. "
Democratic Representative David Cicilline meanwhile called on Twitter to take stronger action against Trump
"The President's Twitter account is posting lies and misinformation at a breathtaking clip," he tweeted. "It is a threat to our democracy and should be suspended until all the votes are counted."
- Fighting misinformation -
Facebook has activated a command center watching the platform and ready to react to misinformation during the vote.
"Our Election Operations Center will continue monitoring a range of issues in real time," said a Facebook statement.
Nonprofit activism group Avaaz said its "war room" was also keeping tabs on Facebook and reported "last-ditch" Spanish-language misinformation, including posts about the prospects of a post-election coup or civil war.
The Election Integrity Partnership research coalition said a Google search for swing states turned up a YouTube video channel that was displaying a fake live feed of election results.
"Thousands of people may have been duped into streaming a fake YouTube video purporting to show election night results," the researchers said in a post.
YouTube removed the video.
Some groups at Facebook were being used to share stories of going to polling places without face masks to "scare liberals away," according to a post by Kayla Gogarty of nonprofit watchdog group Media Matters.
The platforms have pledged to step up scrutiny of false election information, including premature claims of victory, seeking to avoid a repeat of 2016 manipulation efforts.
Over the past days, Facebook and Twitter added disclaimers to Trump posts calling into question the integrity of mail-in ballots.
Twitter last month updated its policy aiming to prevent efforts to manipulate or interfere in elections. That calls for actions against false claims for victory or any incitement to violence.
YouTube has also sought to limit the sharing of videos with election misinformation. Last month it began adding information panels to videos about voting by mail.
Separately, Facebook said it implemented its policy banning political ads after the close of polls.
A Facebook spokesperson said the goal was "reducing the chance for confusion or abuse" and that the ban will likely last about a week.