A post by US President Donald Trump has been given a fact-check label by Twitter for the first time. Mr Trump tweeted, without providing evidence: "There is NO WAY (ZERO!) that Mail-In Ballots will be anything less than substantially fraudulent." Twitter put a warning label in the post and linked to a page that described the claims as "unsubstantiated". Mr Trump on Wednesday threatened to "strongly regulate" or even "close down" social media platforms. He tweeted to his 80 million followers that Republicans felt the platforms "totally silence conservatives" and that he would not allow this to happen. In an earlier tweet, he said that Twitter was "completely stifling free speech". Later on Wednesday he said that Twitter "has now shown everything we have been saying about them... is correct" and vowed "big action to follow". It is unclear what regulatory steps the president could take without new laws passed by Congress. The White House has yet to offer further details. For years, Twitter has faced criticism for not acting on the president's controversial tweets, which include personal attacks on political rivals and debunked conspiracy theories. This month the platform introduced a new policy on misleading information amid the coronavirus pandemic. But recent posts in which Mr Trump - who has more than 80 million followers on Twitter - promoted a conspiracy theory about the death of political aide Lori Klausutis, blaming a high-profile critic, have not received the same treatment.