(Bloomberg) -- Twitter Inc. has started fact-checking Donald Trump. The U.S. President didn’t take it lightly.

Following years of criticism that the social network let the president spread misinformation, on Tuesday a pair of Trump’s tweets that made unsubstantiated claims about mail-in voting were appended with links reading “Get the facts about mail-in ballots.”

The labels take readers to a page with a collection of stories and reporters’ tweets about the president’s claims, as well as an item apparently authored by Twitter staff titled “What you need to know” that rebuts Trump.

It’s the first time Twitter has taken action on Trump’s posts for being misleading, and the president responded shortly afterward, tweeting that the company was interfering with the 2020 election.

“Twitter is completely stifling FREE SPEECH, and I, as President, will not allow it to happen!” Trump posted to his 80 million followers.

On Wednesday morning, Trump came back to the subject, writing in a tweet that “Republicans feel that Social Media Platforms totally silence conservative voices. We will strongly regulate, or close them down, before we can ever allow this to happen.”

Twitter shares fell 1.65% in early trading in New York after Trump’s tweet.

The labels are part of a policy Twitter expanded earlier this month when it started labeling misinformation related to Covid-19. Posting misinformation is not against the company’s rules, but Twitter is adding links providing more information to tweets “where people may still be confused or misled,” it said at the time. Twitter has expanded that policy to include tweets about voting, according to a spokesperson, who declined to share if this policy included other topics.

“Trump falsely claimed that mail-in ballots would lead to ‘a Rigged Election,’” the Twitter-authored item reads. “However, fact-checkers say there is no evidence that mail-in ballots are linked to voter fraud.”

Twitter confirmed it had added the fact-checking links to Trump’s tweets.

Trump campaign manager Brad Parscale said the label was an effort to impede the president’s efforts to get his message to voters. “Partnering with the biased fake news media ‘fact checkers’ is only a smoke screen Twitter is using to try to lend their obvious political tactics some false credibility,” Parscale said in a statement.

Trump shared the same post about mail-in ballots on Tuesday to his Facebook page, where he has more than 29 million followers. That post is still up and doesn’t include any warning or label. Facebook Inc. didn’t immediately respond to a request for comment.

Twitter was under pressure earlier in the day to remove other Trump tweets -- just not the ones focused on mail-in ballots. The New York Times published a letter written by a man whose wife died in former Representative Joe Scarborough’s office, asking Twitter Chief Executive Officer Jack Dorsey to remove tweets Trump posted encouraging a baseless conspiracy theory that Scarborough murdered the woman, Lori Klausutis.

Twitter issued a statement apologizing for the pain Trump’s tweets caused Klausutis’s family but did not say whether the tweets would be removed. They are still visible on the president’s Twitter account and on his Facebook page.

(Updates with Trump tweet on social media in sixth paragraph, shares in seventh)

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