'Black Lives Matter': new protests after another police killing

'Black Lives Matter' is once again a top trending subject on social media, after a black motorist was shot at close range by police in the US.

A video of the killing shows Philando Castile bleeding to death and was viewed online by two million people by Thursday.

A four-year-old girl witnessed the shooting of Philando Castile from the back seat of the car in a town near Minneapolis, Minnesota on Wednesday, as her mother - the victim's girlfriend - livestreamed the shocking scene.

"Oh my God, please don't tell me he's dead, please don't tell me my boyfriend just went like that...," the woman, identified on her Facebook page as Lavish Reynolds and also known as "Diamond," is heard telling a police officer pointing a gun through the car window.

"You shot four bullets into him, sir. He was just getting his license and registration, sir."

Pulled over apparently for a broken tail light, Castile had informed the officer that he was carrying a licensed gun, according to his girlfriend's filmed account.

In the background, an officer is heard shouting: "I told him not to reach for it. I told him to get his hands up."

Shocked family members immediately demanded justice for the 32-year-old Castile, a school cafeteria worker, whose mother described him as a law-abiding citizen who kept out of trouble.

"I think he was just black in the wrong place," Valerie Castile told CNN. "Every day you hear of another black person being shot down, gunned down by the people that are supposed to protect us."

It is the second case of racism-tinged police violence to rock the US in as many days.

A day earlier, a black father of five died in another police shooting captured on video, that time in Louisiana. Alton Sterling was pinned to the ground and shot several times at point blank range, prompting the launch of a federal civil rights investigation.

"We're being hunted every day. It's a silent war against African-American people as a whole," Castile said.

Black lives matter

The debate on police use of lethal force, especially against young black men, is set to hit fever pitch - as a fourth officer goes to trial Thursday in one of the highest-profile such cases of recent years.

Three officers so far have escaped conviction in the case of Freddie Gray, a young black man who died last year in Baltimore after suffering spinal injuries in the back of a police van.

Police said the latest incident was being investigated and a handgun was recovered at the scene.

Philando Castile's girlfriend - clearly in shock - methodically narrates the shooting incident in the 10-minute video as officers can be heard shouting and swearing in the background.

She starts weeping as it becomes clear Castile is dying.

Castile can be seen in the driver seat, large blood stains spreading through his white shirt. He was later taken to hospital and pronounced dead.

"They didn't let me see my son's body, at all," Valerie Castile said on CNN.