An oversight board for Facebook's parent company says the platform was wrong to remove a post with a common protest slogan against Iran's leader.
In July, moderators at Meta said the use of "marg bar Khamenei" (literally "death to Khamenei") broke guidelines.
However, the board said the phrase was often used rhetorically to mean "down with Khamenei" and did not represent a credible threat of violence.
It also issued recommendations "to better protect political speech".
The Oversight Board acts independently to make decisions about what content should be permitted or banned on Meta's platforms, which also include Instagram and WhatsApp.
Millions of Iranians were relying on Meta's social networks for news before authorities began restricting access in September in response to nationwide protests against the clerical establishment and Supreme Leader. Ayatollah Ali Khamenei.
The continuing unrest - during which crowds have been heard chanting the Persian "marg bar Khamenei" - was triggered by the death in custody of a young woman detained by morality police for allegedly wearing her hijab "improperly".
Authorities have portrayed the protests as "riots" and responded with lethal force. So far, at least 519 protesters have been killed and 19,300 arrested, according to the Human Rights Activists' News Agency (HRANA). Four detainees have been executed after what the UN has condemned as "unfair trials based on forced confessions".
"In the Iranian context, the board finds that Meta must do more to respect freedom of expression, and permit the use of rhetorical threats," the Facebook Oversight Board's ruling said.
"The Iranian government systematically represses freedom of expression and digital spaces have become a key forum for dissent. In such situations, it is vital that Meta supports users' voices."
It added that Meta should have anticipated issues around the anti-government protests that took place on Iran's National Day of Hijab and Chastity last July and instructed moderators not to remove content containing the "marg bar Khamenei" slogan.
"As this case shows, its failure to do so led to the silencing of political speech aimed at protecting women's rights."
The board said it had made recommendations "to better protect political speech in critical situations, such as that in Iran, where historic, widespread, protests are being violently suppressed".
It also urged Meta to include in its guidelines on violent content the criteria used to determine when rhetorical threats against heads of state were permitted.
After Russia invaded Ukraine last year, Meta temporarily allowed its users in some countries to call for the death of Russian President Vladimir Putin. It reversed the exemption two days later, after its existence was reported.
https://www.bbc.com/news/world-middle-east-64220937
