Graphic footage has emerged showing a homeless man being shot and killed by police in the US who fired a barrage of 46 bullets as he held a penknife.

Milton Hall, who was mentally ill, was surrounded by eight officers training their guns in a shopping centre car park in Saginaw, Michigan, in July 2012.

The 49-year-old had been arguing with police after an alleged altercation with a shop assistant for several minutes and the video shows him refusing an officer’s demand to put down the knife.

After a tense stand-off, he appeared to step forward and police opened fire. The footage, taken by a bystander, shows Mr Hall fall down almost immediately and lie unmoving on the ground.

He had been shot 14 times.

As he lies bleeding, the officers are seen attempting to handcuff his lifeless arms and dragging his body along the ground, with one officer appearing to kick his back.

Police officers attempting to handcuff Mr Hall after shooting him (American Civil Liberties Union)

Shocked onlookers can be heard shouting at police after the shooting, with one asking: “Why did they have to shoot him so many times?”.

The death of Mr Hall, who was black, at the hands of white police officers sparked protests and calls for the police who shot him to face criminal charges.

But the county prosecutor and Department of Justice declined to bring charges against them, accepting the that the action was justified in the face of what they felt was a threat.

Mr Hall was “known” to authorities for committing previous offences, a police spokesperson said at the time, but his family said he had only committed minor, non-violent crimes.

His mother, Jewel Hall, described her son’s death as “an assassination”, by a “firing squad dressed in uniforms”.

Milton Hall's mother, Jewel Hall (American Civil Liberties Union)

In an interview with the American Civil Liberties Union, who released the latest footage, she said Mr Hall fought for equal rights and worked with Rosa Parks, the famous civil rights activist whose refusal to give up her seat for a white bus passenger in 1955 was one of the key events in the civil rights movement.

“His blood [was] running down the street like water,” Mrs Hall said. “And he wasn't a threat, I mean, he had a little pen knife.

“He had no idea that those policemen would do that to him…justice still has not been served.”