Twitter and Facebook have both flagged posts by US President Donald Trump that claimed a Supreme Court decision on mail-in voting in Pennsylvania would lead to "rampant" fraud and was "very dangerous."
Twitter hid the president's tweet, sent late on Monday the day before the US presidential election, behind a label which said the content was "disputed" and "might be misleading."
Mr Trump and his Republican allies have repeatedly said, without evidence, that mail-in votes are prone to fraud, although election experts say that is rare in US elections. Mr Trump's tweet also said the Supreme Court's decision would "induce violence in the streets."
Social media companies are under pressure to curb misinformation on their platforms ahead of the US election. Twitter has labeled or put warnings on tweets from Mr Trump multiple times this year for violating its rules.
Twitter also prevented users from retweeting or replying to the post, allowing only 'quote tweets.' It said the tweets will also not be algorithmically recommended by its systems.
The Election Integrity Partnership, who tweeted that Twitter took action about 40 minutes after Mr Trump's tweet was sent, said it would "be wise" to pick up the pace.
"Twitter's action effectively stopped the tweet's spread. However, it had already been retweeted 55K+ times and favorited 126K+ times. This is as much or more reach than most other tweets put out by @realDonaldTrump today," the research group tweeted.
The US Supreme Court last week allowed extended deadlines for receiving mail-in ballots in Tuesday's election in Pennsylvania and North Carolina, states pivotal to Mr Trump's re-election chances.
The decision let stand a ruling by Pennsylvania's top court allowing mail-in ballots postmarked by Election Day and received up to three days later to be counted.
Facebook added a disclaimer to Mr Trump's post, which had been shared 4,200 times, saying that voting by mail and voting in person have a "history of trustworthiness" in the United States, with voter fraud being extremely rare.
Facebook also labeled a Fox News video posted by Mr Trump in which he talked about "cheating" in Pennsylvania with the same message.
