A man who was among the onlookers shouting at a Minneapolis police officer to get off George Floyd last May has told a court he called 911 after paramedics took Mr Floyd away, “because I believed I witnessed a murder”.

Donald Williams, a former wrestler who said he was trained in mixed martial arts, including chokeholds, returned to the witness box a day after describing seeing Mr Floyd struggle for air and his eyes roll back into his head.

He said he watched Mr Floyd “slowly fade away… like a fish in a bag”.

Prosecutor Matthew Frank played back Mr Williams’ emergency call, on which he is heard identifying Derek Chauvin by his badge number and telling the dispatcher the accused officer had kept his knee on Mr Floyd’s neck despite warnings that his life was in danger. She offers to switch him to a sergeant.

As he is being switched, Mr Williams can he heard yelling at the officers at the scene, “Y’all is murderers, bro!”

On Monday, Mr Williams said he thought Chauvin used a shimmying motion several times to increase the pressure on Mr Floyd. He said he shouted to the officer that he was cutting off Mr Floyd’s blood supply.

Mr Williams recalled that Floyd’s voice grew thicker as his breathing became more laboured, and he eventually stopped moving.

During cross-examination Tuesday, Chauvin lawyer Eric Nelson sought to show that Chauvin and his fellow officers found themselves in an increasingly tense and distracting situation, with the crowd of onlookers getting agitated over Mr Floyd’s treatment.

Mr Nelson said Mr Williams seemed to become increasingly angry at police on the scene, swearing at and taunting Chauvin with “tough guy”, “bum” and other names, then calling him expletives, which the lawyer repeated in court.

Mr Williams initially admitted he was getting angrier, but backtracked and said he was controlled and professional and was pleading for Mr Floyd’s life but was not being heard.

He said he was stepping on and off the kerb, and at one point, Officer Tou Thao, who was controlling the crowd, put his hand on his chest. Mr Williams admitted under questioning that he told Mr Thao he would beat the officers if he touched him again.

He was among the first prosecution witnesses as Chauvin, 45, went on trial on charges of murder and manslaughter.

The death of the black man after he was held down by the white officer sparked sometimes violent protests around the world and a reckoning over racism and police brutality.

Prosecutors led their case by playing part of the harrowing bystander video of Mr Floyd’s arrest. Chauvin and three other officers were fired soon after the footage became public.

Prosecutor Jerry Blackwell showed the jurors the video after telling them the number to remember was nine minutes, 29 seconds — the amount of time Chauvin had Mr Floyd pinned to the pavement “until the very life was squeezed out of him”.

Mr Nelson countered by arguing: “Derek Chauvin did exactly what he had been trained to do over his 19-year career.”

Mr Floyd was fighting efforts to put him in a patrol car as the crowd of onlookers around Chauvin and his fellow officers grew and became increasingly hostile, Mr Nelson said.

The defence lawyer also disputed that Chauvin was to blame for Mr Floyd’s death.

The 46-year-old had none of the telltale signs of asphyxiation and had fentanyl and methamphetamine in his system, Mr Nelson said. He said Mr Floyd’s drug use, combined with his heart disease, high blood pressure and the adrenaline flowing through his body, caused a heart rhythm disturbance that killed him.

Mr Blackwell rejected that argument, saying it was the officer’s knee that killed him.