Facebook Inc's decisions to allow controversial posts by US President Donald Trump established a "terrible precedent" that could allow the platform to be "weaponized to suppress voting", an external civil rights audit has found.

The report, which Facebook commissioned two years ago, said the social network has not done enough to protect users from discrimination, falsehoods and incitement to violence, adding to pressure on the company in the midst of an advertiser boycott.

The findings come at a time when more than 900 advertisers, including major brands such as Coca-Cola and Unilever, have joined a boycott started by major United States civil rights groups, including the Anti-Defamation League and the NAACP, to pressure Facebook to take concrete steps to block hate speech.

"Many in the civil rights community have become disheartened, frustrated and angry after years of engagement where they implored the company to do more to advance equality and fight discrimination, while also safeguarding free expression," the auditors wrote.

Facebook has taken a hands-off approach to political speech compared with rivals, notably leaving untouched posts by Trump in recent weeks which were flagged by its rival Twitter for falsehoods and incitement of violence.

SOURCE: Reuters news agency