Thousands of ads from Joe Biden's election campaign and some from President Trump's have been mistakenly blocked by Facebook.
The social network's policy to ban any new ads a week before election day as part of its crackdown on misinformation kicked in on Tuesday, but pre-approved ads should still run.
Mr Biden's digital campaign manager expressed fury at the situation.
Facebook blamed technical issues, and apologised.
In a blog, it said: "We have identified a number of unanticipated issues affecting campaigns of both political parties.
"Some were technical problems. Others were because advertisers did not understand the instructions we provided and when and how to make changes to ad targeting."
"While this impacted a small proportion of the ads about politics and social issues in our system, we regret any disruption in the delivery of these ads."
Facebook admitted it could have been clearer in explaining how advertisers would need to edit their targeting during the restricted period.
In a series of tweets, Mr Biden's digital director Rob Flaherty expressed his anger that ads were being blocked in the crucial final week of campaigning.
In a statement, he described the situation as a "fiasco" leading to "widespread issues that are plaguing all of our ad campaigns since the onset of new ad restrictions".
He added: "It is abundantly clear that Facebook was wholly unprepared to handle this election despite having four years to prepare."
A thing worth highlighting here is that, like, 80% of the problems with facebook are on the organic side, not the ads side. This ads policy was largely a distraction, but they *still* are finding a way to get it wrong. https://t.co/y0cnwPabXG— Rob Flaherty (@Rob_Flaherty) October 30, 2020
President Trump's campaign confirmed that some of its ads were also blocked.
Facebook's record on handling political ads has been controversial, and it has been criticised for its decision not to fact-check posts by politicians.
Responding to pressure to do more about misinformation, it decided to ban ads in the week leading up to the US election.
Later, it said it would ban ads which prematurely declared victory, and then added that it would also temporarily block political ads after the polls closed.