As onlookers pleaded with Minneapolis police Officer Derek Chauvin to take his knee off George Floyd’s neck, the officer just gave them a “cold” and “heartless” stare, the teenager who shot the video of the arrest has told a court.

In sometimes-tearful testimony, Darnella Frazier, 18, said Chauvin continued to kneel on Mr Floyd’s neck and fellow officer Tou Thao would not let onlookers get close, even when one of them identified herself as a firefighter and begged repeatedly to check Mr Floyd’s pulse.

“They definitely put their hands on the Mace, and we all pulled back,” Ms Frazier told the jury.

Ms Frazier said of Chauvin: “He just stared at us, looked at us. He had like this cold look, heartless. He didn’t care. It seemed as if he didn’t care what we were saying.”

Mr Floyd’s death last May, along with the video of the black man pleading that he could not breathe and onlookers angrily yelling at the white officer to get off him, triggered sometimes-violent protests around the world and a reckoning over racism and police brutality across the US

Ms Frazier testified that she began recording the scene because “it wasn’t right, he was suffering, he was in pain”.

She said she had walked to a convenience store with her nine-year-old cousin when she came upon the officers, and sent the girl inside because she did not want her to see “a man terrified, scared, begging for his life”.

Ms Frazier breathed heavily and wept as she viewed pictures of Chauvin kneeling on Mr Floyd and after a prosecutor asked her to describe how the encounter changed her life.

She said she looks at her father and other black men in her life, and “how that could have been one of them”.

“I stay up at night apologising to George Floyd for not doing more… not saving his life,” she said, adding of Chauvin: “It’s not what I should have done, it’s what he should have done.”

Chauvin attorney Eric Nelson sought repeatedly to show that Chauvin and his fellow officers found themselves in an increasingly tense and distracting situation, with the growing crowd of onlookers becoming agitated and menacing over Mr Floyd’s treatment.

Under cross-examination from Mr Nelson, Ms Frazier said bystanders became increasingly upset by what they were seeing and got louder and louder, “more so as he was becoming more unresponsive”.

But when Ms Frazier was asked by a prosecutor whether she saw violence anywhere on the scene, she replied: “Yes, from the cops. From Chauvin, and from officer Thao.”

Chauvin, 45, is charged with murder and manslaughter, accused of killing Mr Floyd by pinning the 46-year-old handcuffed man to the pavement for what prosecutors said was nine minutes and 29 seconds.

Mr Floyd was arrested after being accused of trying to pass a counterfeit 20 dollar bill at the convenience store.

The most serious charge against the now-fired officer carries up to 40 years in prison.

The defence has argued that Chauvin did what his training told him to do and that Mr Floyd’s death was not caused by the officer but by a combination of illegal drug use, heart disease, high blood pressure and the adrenaline flowing through his body.