The move came a day after US President Donald Trump used the first televised debate with Democratic challenger Joe Biden to amplify his claims that the November 3 presidential election will be "rigged."
Facebook Inc has banned ads on its flagship website and Instagram photo and video sharing service that claim widespread voting fraud, suggest US election results would be invalid, or which attack any method of voting.
The company announced the new rules on Wednesday in a blog post, adding to earlier restrictions on premature claims of election victory.
US President Trump has been especially critical of mail-in ballots, and he cited a number of small unrelated incidents to argue that large scale voting fraud is already happening.
Facebook under pressure
Facebook has been under fire for refusing to fact-check political ads more broadly and for rampant organic misinformation.
Citing hate speech rules, it also moved on Wednesday to remove Trump campaign ads suggesting that immigrants could be a significant source of coronavirus infections.
Facebook said the new election ad prohibition would include those that "portray voting or census participation as useless/meaningless" or that "delegitimise any lawful method or process of voting or voting tabulation ... as illegal, inherently fraudulent or corrupt."
Facebook also cited ads that call an election fraudulent or corrupt because the result was unclear on election night or because ballots received afterwards were still being counted.
Wow! 100,000 Mail In Ballots in New York City a total MESS. Mayor and Governor have no idea what to do. Big Fraud, Unfixable! Cancel Ballots and go out and VOTE, just like in past decades, when there were no problems!— Donald J. Trump (@realDonaldTrump) September 30, 2020