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A coalition of more than 70 civil rights groups has urged Facebook to be clearer about the content it removes.

In a to Mark Zuckerberg, the group accused the social network of "disproportionate censorship of Facebook users of colour".

The coalition wants the site to specify which rule a deleted post has broken, and provide an appeal process.

Facebook has previously responded to a letter submitted by the group, but has not yet replied to the latest one.

The social network sometimes removes content that has been reported or flagged by members, if it is judged to have broken the website's guidelines.

But the coalition argued that the moderation was "racially biased".

"Activists in the Movement for Black Lives have routinely reported the takedown of images discussing racism and during protests, with the justification that it violates Facebook's community standards," the coalition wrote.

"At the same time, harassment and threats directed at activists based on their race, religion, and sexual orientation is thriving on Facebook.

"Your recent response indicates you are adequately addressing the problem. We disagree."

The social network has faced a variety of complaints over perceived censorship of content such as political views, nudity and historical photographs.

The coalition behind the letter includes organisations such as the American Civil Liberties Union, Color of Change, and the Center for Media Justice.

It said it wanted Facebook to:

In a statement, Facebook said: "We have received the letter and are reviewing it."