Facebook is embroiled in another censorship scandal after leaked documents reveal the company will protect white men but not black children.

Training documents for Facebook moderators show the social network will allow hate speech against minority groups such as female drivers and black children, while blocking that against white men.

The documents, obtained by investigative journalism site ProPublica, outline Facebook's guidelines for distinguishing hate speech from political expression. They reveal the unequal way the social network treats threatening and abusive language towards different ethnic, religious and social groups.

One example is a training slide asking moderators "Which of the below subsets do we protect?" The correct answer to the question was "white men", not the "female drivers" and "black children" who also appeared.

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The reason, Facebook said, is because it protects personal characteristics such as sex, race, religious affiliation, nationality and sexual orientation, but doesn't protect those such as political ideology, occupation, age and social class. In the above instance, "drivers" and "children" prevent the groups from being protected.

"The policies do not always lead to perfect outcomes," Monica Bickert, head of global policy management at Facebook, told ProPublica. "That is the reality of having policies that apply to a global community where people around the world are going to have very different ideas about what is OK to share."

Facebook has an army of 4,500 moderators policing material generated by its 2 billion global users with one handbook created in Silicon Valley. It has been criticised for overlooking cultural and national rules in favour of its own guidance.

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The files also show how the site appears to favour members of the elite and government over activists and minorities, ProPublica reports.

In one incident moderators allowed comments from a US politician in the wake of the London terrorist attack that said of radicalised Muslims, "Hunt them, identify them and kill them".

But they blocked a post from a Black Lives Matter activist and Boston poet that said, "All white people are racist. Start from this reference point, or you've already failed." The activist's account was disabled for a week.

The revelation comes after a trove of documents leaked last month showed Facebook's policies for taking down content. They showed its moderators can leave gruesome and violent material on the site, often to raise awareness.