The social media site said the ban will last for at least a week, and it will also prevent political ads in the week leading up to Election Day.
The new safeguards for the 2020 US elections make Facebook better prepared to deal with candidates who prematurely declare victory or contest official results and the possibility of voter intimidation by alleged – and potentially armed – “poll watchers”, the company said.
In addition, posts that cast doubt on election results will be labelled with links to official information.
The social network giant has already banned messages that promote carrying of weapons to polling places or that attempt to organise “coordinated interference” with voting.
Now it will also ban the use of “militarised language” in connection with calls for poll-watching.
Under the new policy, which is not retroactive, US President Donald Trump will not be able to encourage supporters to be unofficial poll watchers as part of an “army for Trump”, as he said in a tweet this week.
The changes – less than three weeks before Election Day and with voting happening already – show how quickly social media companies must respond to existing and anticipated threats to the election process.
Mr Trump’s campaign and its Republican allies have been going to new lengths to contest election procedures and question the integrity of postal ballots.
The president has also refused to commit to a peaceful transfer of power if he loses, though both parties have rejected his comments and vowed a peaceful transition.
Facebook said it has been preparing for this election since 2016, when its platform was used by Russian agents trying to interfere with the US political process.
It also noted that more than 200 elections have taken place worldwide since then.
While Facebook and other social media companies have strengthened their policies and enforcement since, the threats to the election process have also become more sophisticated in that time.
On Tuesday, Facebook began banning groups that openly support QAnon, the baseless conspiracy theory that paints Mr Trump as a secret warrior against a supposed child-trafficking ring run by celebrities and “deep state” government officials.
