Twitter has labelled two of Donald Trump's tweets "unsubstantiated" and accused him of making false claims, a first for the social network which has long resisted calls to censure the US president over some of his questionable posts.
The move drew a furious response from Mr Trump, who used the platform to accuse Twitter of "interfering in the 2020 Presidential Election."
"Twitter is completely stifling FREE SPEECH, and I, as President, will not allow it to happen!" he tweeted.
The social media giant targeted two tweets Mr Trump posted yesterday in which he contended without evidence that postal voting would lead to fraud and a "Rigged Election."
Under the tweets, Twitter posted a link which read "Get the facts about mail-in ballots" and which took users to a notice calling the claims "unsubstantiated", citing reporting by CNN, the Washington Post and other media.
"Trump falsely claimed that mail-in ballots would lead to 'Rigged Election'," the notice contended.
"However, fact-checkers say there is no evidence that mail-in ballots are linked to voter fraud."
.@Twitter is now interfering in the 2020 Presidential Election. They are saying my statement on Mail-In Ballots, which will lead to massive corruption and fraud, is incorrect, based on fact-checking by Fake News CNN and the Amazon Washington Post....— Donald J. Trump (@realDonaldTrump) May 26, 2020
....Twitter is completely stifling FREE SPEECH, and I, as President, will not allow it to happen!— Donald J. Trump (@realDonaldTrump) May 26, 2020
Mr Trump aimed the misleading tweets at California, contending falsely that anyone living in the state would be sent ballots when in fact they will only go to registered voters, according to the notice.
There is NO WAY (ZERO!) that Mail-In Ballots will be anything less than substantially fraudulent. Mail boxes will be robbed, ballots will be forged & even illegally printed out & fraudulently signed. The Governor of California is sending Ballots to millions of people, anyone.....— Donald J. Trump (@realDonaldTrump) May 26, 2020
....living in the state, no matter who they are or how they got there, will get one. That will be followed up with professionals telling all of these people, many of whom have never even thought of voting before, how, and for whom, to vote. This will be a Rigged Election. No way!— Donald J. Trump (@realDonaldTrump) May 26, 2020
He has long used Twitter as a platform to spread abuse, conspiracy theories, false information and insults to his 80 million followers.
For years before being elected in 2016, he built his political brand by supporting the "birther" lie that Barack Obama was not born in the United States and therefore was not eligible to be president.
And yesterday he ignited a storm with an attempted character assassination of MSNBC host Joe Scarborough by spreading the baseless rumour he murdered an aide.
The opening of a Cold Case against Psycho Joe Scarborough was not a Donald Trump original thought, this has been going on for years, long before I joined the chorus. In 2016 when Joe & his wacky future ex-wife, Mika, would endlessly interview me, I would always be thinking....— Donald J. Trump (@realDonaldTrump) May 26, 2020
....about whether or not Joe could have done such a horrible thing? Maybe or maybe not, but I find Joe to be a total Nut Job, and I knew him well, far better than most. So many unanswered & obvious questions, but I won't bring them up now! Law enforcement eventually will?— Donald J. Trump (@realDonaldTrump) May 26, 2020
Twitter, perhaps fearing a clash with one of its most influential users, had previously held out against calls to act.
The tweets in question violated a recently expanded Twitter policy, according to the San Francisco-based company.
"In serving the public conversation, our goal is to make it easy to find credible information on Twitter and to limit the spread of potentially harmful and misleading content", head of site integrity Yoel Roth and global public policy director Nick Pickles said when the change was announced.
Twitter's decision comes as Mr Trump, already facing US economic calamity and 100,000 deaths from coronavirus as well as sinking reelection polls, received a storm of backlash over his pushing of the Scarborough conspiracy theory.
The entirely evidence-free story claims that Mr Scarborough killed a woman he was having an affair with in 2001, when he was a Republican congressman and she was one of his staffers.
Mr Trump pushed the story over the weekend. Yesterday he tweeted: "The opening of a Cold Case against Psycho Joe Scarborough".
"So many unanswered & obvious questions, but I won't bring them up now! Law enforcement eventually will?" he wrote.
The woman who died, Lori Klausutis, was found by investigators to have died after hitting her head during a fall in Mr Scarborough's office, triggered by an abnormal heart rhythm.
Mr Scarborough went on to become a prominent media personality, strongly critical of Mr Trump, and is co-host of the 'Morning Joe' show on MSNBC with his wife Mika Brzezinski, whom Mr Trump calls "low I.Q. Crazy Mika".
Ms Klausutis's widower, Timothy Klausutis, wrote to Twitter CEO Jack Dorsey, pleading with him to delete Mr Trump's "vicious lie".
"I'm asking you to intervene in this instance because the President of the United States has taken something that does not belong to him - the memory of my dead wife and perverted it for perceived political gain," he wrote in a letter published by The New York Times.
When asked about the letter, Mr Trump told reporters at the White House: "I'm sure ultimately they want to get to the bottom of it and it's a very serious situation."
He added: "As you know, there is no statute of limitations."
Democratic presidential candidate Joe Biden said during a CNN interview yesterday that Twitter and other social media platforms should "say it's not true" when misleading statements are broadcast.
Asked about the fallout from the Scarborough tweets, a Twitter spokesman said: "We are deeply sorry about the pain these statements, and the attention they are drawing, are causing the family.
"We've been working to expand existing product features and policies so we can more effectively address things like this going forward, and we hope to have those changes in place shortly."