A US policeman has been stood down for threatening to kill a peaceful protester in Ferguson, the suburb in St Louis, Missouri, that has been rocked by unrest since an unarmed black teenager was shot dead by a white police officer this month.

Lieutenant Ray Albers was caught on film pointing an automatic rifle at a protester in a dark street on Tuesday night, shouting "I will f---ing kill you. Get back, get back."

When another protester asked his name, Lieutenant Albers shouted: "Go f--- yourself."

"Your name is 'Go f--- yourself'?" the protester taunted. "Okay, Go f--- yourself. Hello, officer Go- f--- yourself."

The Missouri American Civil Liberties Union sent a letter to Missouri police arguing the officer had heightened tensions in the suburb.

"Please take immediate action to identify the officer in question and remove him from further duties in Ferguson," the union demanded.

In a statement released on Wednesday, the St Louis County Police said a sergeant "immediately took action, forcing the officer to lower the weapon, and escorting him away from the area".

"The St Ann Police Officer involved in the incident has been relieved of duty and suspended indefinitely," the statement said.

The international online network Anonymous claimed to have released a file containing Lieutenant Albers' name, age, date of birth, social security number, credit report and phone number.

On Twitter, the hastag #officergof---yourself accompanied links to the video and words of encouragement for the livestreaming community that has documented the protests.

A fake account, @ofgofuckyourself, drew attention to the fact that Lieutenant Albers had not been fired, only suspended.

Other Twitter users questioned why Lieutenant Albers had been suspended while Officer Darren Wilson, who killed Michael Brown, was on paid leave.

The incident has added to tensions that have riven the majority-black community since Mr Wilson fired six shots into Mr Brown on August 9.

Missouri Governor Jay Nixon replaced St Louis police with state police and imposed a curfew to curb sometimes violent protests. 

Say hello to everyone's favorite cop, the infamous Officer "Go Fuck Yourself" aka Ray Albers - http://t.co/pvIUSLk2Bl pic.twitter.com/ATrYEHgjAl — Operation Ferguson (@OpFerguson) August 20, 2014

. @GovJayNixon Look how ur 'peacekeepers' speak 2 ppl #OfficerGoFuckYourself Threatens to Kill Ferguson Livestreamers http://t.co/RPM8u0xEJu — liberalease (@liberalease) August 20, 2014