Police officer quits after he was caught holding gun to homeless man's head as he was handcuffed on floor

A police officer has quit after shocking footage emerged showing him holding a rifle to the head of a handcuffed homeless man, who was screaming in pain.

The bodycam video was taken in Grand Rapids, Michigan, as Officer Kevin Penn responded to a call about an attempted armed robbery.

The suspect, 28-year-old Javonte Howell, had already been tasered and was in handcuffs when Penn pressed the weapon to his head.

Another officer was also pointing a gun at Howell.

Penn told the suspect: "This is a gun, if you move, you possibly could be injured, do not move, does that make sense?"