Twitter Inc. on Tuesday issued a fact-check warning on President Donald Trump's tweets that called mail-in ballots "fraudulent," angering the U.S. leader who has used social media as a powerful tool to attack opponents and get his message across to supporters.

It is the first time Twitter has used a blue exclamation mark, advising users "get the facts," below tweets sent by Trump, who has more than 80 million followers on the platform.

A spokesperson for Twitter said it took action based on a system introduced this month, because the tweets "contain potentially misleading information about voting processes."

At issue are two of Trump's tweets posted earlier in the day, in which he said mail-in ballots will be "substantially fraudulent" and will lead to a "Rigged" presidential election in November. Calls for postal voting have been growing inside the opposition Democratic Party amid the coronavirus pandemic.

Twitter's advisory directs users to a page citing news articles and other information. A summary of the information says "fact-checkers say there is no evidence that mail-in ballots are linked to voter fraud" and that Trump is making false claims.

Trump angrily reacted by posting new messages, saying Twitter is "now interfering in the 2020 Presidential Election" and condemning the company for carrying out "fact-checking by Fake News" through the likes of CNN and The Washington Post.

"Twitter is completely stifling FREE SPEECH, and I, as President, will not allow it to happen!" he tweeted.

The 73-year-old Trump, a businessman and TV personality before becoming president, is known for his strong distrust of mainstream news media which he calls "fake news," and has preferred Twitter as a means of communicating with the populace.