George Floyd ask for his mother as he died under the knee of a police officer who used a "blood choke" to kill him, a court heard.
Derek Chauvin, 45, is accused of murdering Floyd by kneeling on his neck for nearly nine minutes outside a grocery store in the city of Minnesota, US, last May.
Donald Williams, a professional mixed martial arts fighter, can be heard on the videos of the arrest of Floyd screaming insults at Chauvin and demanding the police check for Floyd's pulse.
He told jurors at the trial on Tuesday he believed Chauvin was using his knee in a "blood choke" on Floyd, a wrestling move to knock an opponent unconscious, and a "shimmy" move to tighten pressure on Floyd's neck.
"You can see that he's trying to gasp for air," Williams, 33, said of Floyd.
A 911 call Williams made after the arrest was played.
Teenager Darnella Frazier shot the viral clip in which Mr Floyd can he can be heard saying "I can't breathe" that triggered a wave of protests around the world in response to the unarmed black man's death.
Her voice shook with emotion as she took to the stand to give witness testimony on the second day of the murder trial of the former Minneapolis police officer accused of killing Floyd, 46.
Prosecutors played videos of the arrest to the jury taken from multiple angles, including the teenager's video that shows Chauvin pressing his knee into the neck of a dying Floyd.
In opening arguments on Monday, prosecutors said the clips showed excessive force.
The defence told the jury use of force is a necessary part of policing and Chauvin was following his training and is not guilty of the charges of second-degree murder, third-degree murder or second-degree manslaughter.
Video of Darnella's testimony was not broadcast on Tuesday as she was 17 at the time she captured footage of Floyd being pinned down.
Audio of the tearful teen in court recounting the moment Floyd was pinned down by officers outside Cup Foods grocery story was recorded instead.
She wept and her voice grew shaky as she was shown an image from CCTV video at the trial.
The now- 18-year-old told the jury she was walking past Cup Foods with her cousin when she saw Floyd being restrained.
A worker at the grocery store had moments before accused Floyd of using a fake $20 bill.
Darnella told the jury she saw police arresting him outside and pulled out her cellphone.
"I see a man on the ground and I see a cop kneeling down on him," she testified, explaining why she first made sure her young cousin had safely gone inside the store, out of sight.
"A man terrified, scared, begging for his life."
The young woman's voice audibly shook as she was shown the moment when Chauvin, his knee on Mr Floyd's neck, appears to look directly into her camera lens.
"Yes," she said, crying and catching her breath when asked if she recognised him. "
This was the officer that was kneeling on George Floyd's neck."
She told the court of witnessing Mr Floyd struggling for breath.
The teen told jurors: "I heard George Floyd saying 'I can't breathe, please, get off of me, I can't breathe'."
"He cried for his mom. He was in pain. It seemed like he knew... it seemed like he knew it was over for him.
"He was terrified, he was suffering - this was a cry for help, definitely."
Chauvin's lawyers have said that he was distracted from "the care" of Mr Floyd by the angry bystanders that joined Frazier on the sidewalk, who could be heard shouting in one of the clips shown to the court.
Prosecutors asked her whether he heard any bystanders threaten the police, and she said no.
"Would you describe yourselves as an unruly mob?" Jerry Blackwell, a prosecutor, asked her.
"No," Darnella said, adding the only person she saw being violent was Chauvin.
The trial continues.
