Facebook's independent oversight board announced on Wednesday that it will maintain the ban of former US president Donald Trump from the platform but called for a further review of the case within six months.
The board said Mr Trump "created an environment where a serious risk of violence was possible" with his comments on the January 6 riot at the US Capitol.
"Given the seriousness of the violations and the ongoing risk of violence, Facebook was justified in suspending Mr Trump's accounts on January 6 and extending that suspension on January 7," the board said after its review.
But the panel added that "it was not appropriate for Facebook to impose the indeterminate and standardless penalty of indefinite suspension" and called for the platform to "review this matter to determine and justify a proportionate response" within six months.
"It is not permissible for Facebook to keep a user off the platform for an undefined period, with no criteria for when or whether the account will be restored," the review board said in its written opinion.
"What Facebook, Twitter and Google have done is a total disgrace and an embarrassment to our country," Mr Trump said in response to the decision.
The former president released a statement following Facebook's announcement in which he claimed that he has been silenced for "speaking the truth" and that the decision infringes on his right to free speech.
The ruling could be a defining moment for the leading social network's "supreme court", envisioned by company founder Mark Zuckerberg to decide what to allow or remove from Facebook.
The oversight board's decisions are binding on Facebook and cannot be appealed.
"This is a huge decision. It's getting a lot of attention and deservedly so," said Daniel Kreiss, University of North Carolina professor and researcher in politics and social media.
"This is significant for the global precedent it will set. If they uphold the ruling, I think you will see more robust enforcement around the world."
The oversight panel consists of jurists, policy experts, journalists and others from around the world.
Social platforms are struggling to remain open to political discourse while filtering out misinformation, abusive comments and incitements to violence.
Mr Trump was suspended from Facebook after he posted a video during the deadly January 6 rampage at the US Capitol by his supporters in which he stated, "We love you, you're very special."
He was banned permanently by Facebook the next day, and taken off other platforms including Twitter and YouTube.
Some analysts said Facebook and other social networks should have acted on Mr Trump sooner, after years of his exemption from rules on hateful content because of his "newsworthiness" as a political leader.
"He was using Facebook and other platforms to actively spread patently false content about electoral processes, very effectively undermining US democracy," said Samuel Woolley, a University of Texas professor specialising in online propaganda.
Facebook referred the case to the oversight board, in line with its position that company executives should not be in the position of making key decisions on content and political speech.
The panel has received more than 9,000 comments on the case.
But the move by Facebook and others has also drawn a torrent of criticism from Trump supporters, who argue that large tech platforms are biased and stifling opposing views.
And the ban has also sparked concern from others including German Chancellor Angela Merkel, who called Facebook's move "problematic", and from civil liberties activists.
Jameel Jaffer, executive director of Columbia University's Knight First Amendment Institute, said the issue was more complex than evaluating Mr Trump's comments.
"I'm hopeful the board will use this case as an opportunity to put a spotlight on Facebook's decisions about the design of its platform," Mr Jaffer said.
"These engineering decisions are often invisible but they determine which speech proliferates on the platform, how quickly it spreads, who sees it and in what context they see it."
In its submission to the board, the institute said Facebook should conduct "an independent study of how its platform may have contributed to the events of January 6".
It said the panel should rule on Mr Trump "only after the company has provided it with the results of that study".
