Facebook says it will ban political advertising the week before the US election, one of its most sweeping moves against disinformation yet as CEO Mark Zuckerberg warned of a "risk of civil unrest" after the vote.

The social media giant also vowed to fact check any premature claims of victory, stating that if a candidate tries to declare himself the winner before final votes are tallied "we'll add a label to their posts directing people to the official results."

And it promised to "add an informational label" to any content seeking to delegitimize the results or claim that "lawful voting methods" will lead to fraud.

"I'm concerned about the challenges people could face when voting. I'm also worried that with our nation so divided and election results potentially taking days or even weeks to be finalized, there could be an increased risk of civil unrest across the country," Zuckerberg said in a post.

Democrats have long warned that President Donald Trump and his supporters may try to sow chaos with false claims on November 3, when the vote will take place amid unprecedented health and economic crises, social unrest and protests for racial justice.

The US remains the epicenter of the world's worst coronavirus outbreak, and voters are expected to shift to mail-in voting, with an estimated three-quarters of the population eligible to do so. 

As a result officials are warning that the final tally may not be revealed until well after voting day -- spurring fears that paranoia and rumor-mongering could hit an all-time high.

Trump -- a prolific user of social media who is trailing Democratic challenger Joe Biden in the polls -- has recently hurtled down a rabbit hole of conspiracy theories filled with claims that he is victim of a coup and/or plans to rig the polls.

Almost daily, he claims that increased mail-in voting is a gambit to "rig" the election against him, and he has refused to say whether he will accept the results. 

He has also opposed more funding for the cash-strapped US Postal Service (USPS), acknowledging the money would be used to help process ballots.

And he has refused to condemn the presence of armed vigilantes in the streets during a wave of social justice protests across America this summer, spurring fears of unrest if there is not a clear result immediately after November 3. 

Opponents say Trump's increasingly extreme resistance to expanded mail-in voting -- a method already used widely in the United States -- is an attempt to suppress voter turnout, while setting up an excuse to challenge the result if he is defeated.

"This election is not going to be business as usual," Zuckerberg, who has come under increasing pressure to do more to combat conspiracy theories at Facebook, said.  

"We all have a responsibility to protect our democracy. That means helping people register and vote, clearing up confusion about how this election will work, and taking steps to reduce the chances of violence and unrest."