Twitter on Tuesday for the first time prompted readers to check the facts in tweets sent by US President Donald Trump and accused him of making false claims.

Much has been made about the role of social media in politics, and that was in all caps on Tuesday as Twitter contested US President Donald Trump's exhortations about mail-in election ballots, a voting trend magnified by the coronavirus pandemic.

For the first time, Twitter prompted readers to fact-check Trump's tweets about mail-in ballots.

Twitter's blue exclamation mark suggested readers "get the facts about mail-in ballots" and directed them to a page with news articles and information about the claims aggregated by staffers of the San Francisco-based company.

Trump tweeted back on Tuesday evening that "@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.... Twitter is completely stifling FREE SPEECH, and I, as President, will not allow it to happen!"

The Washington Post is owned by Jeff Bezos, the founder and CEO of e-commerce giant Amazon Inc.

Trump, who has more than 80 million followers on Twitter, tweeted earlier in the day that mail-in ballots would be "substantially fraudulent" and result in a "rigged election". He also singled out California Governor Gavin Newsom, although the state is not the only one that uses mail-in ballots.

Trump and many conservative politicians and commentators have long accused social media companies of stifling their voices in favor of Democrats. They also have expressed concerns about "ballot harvesting" in the mail-in election process, contending that some votes are not properly awarded to a particular candidate.

Trump has long relied on Twitter to get his message out to supporters and voters and has regularly accused US mainstream media of unfair coverage.

"Trump makes the unsubstantiated claim that mail-in ballots will lead to voter fraud," said a headline at the top of the page, followed by a "what you need to know" section correcting three false or misleading claims made in the tweets.

Twitter confirmed that it was the first time it had applied a fact-checking label to a tweet by the president, in an extension of its new "misleading information" policy introduced this month to challenge misinformation about the coronavirus.