The teenager who took the viral video of George Floyd's death has told the trial of the white police officer charged with killing the 46-year-old black man that she regrets not being able to save his life.

Darnella Frazier, 18, was among the witnesses who provided emotional testimony today at the high-profile trial of former Minneapolis police officer Derek Chauvin.

Mr Chauvin, 44, is charged with murder and manslaughter for his role in Floyd's death last May, which was captured on video by Ms Frazier and seen by millions, sparking anti-racism protests around the globe.

In the video, Mr Chauvin is seen kneeling on Mr Floyd's neck for more than nine minutes as the handcuffed Floyd complains that he cannot breathe and eventually falls unconscious.

"I have a black father. I have a black brother," Ms Frazier told the court with tears running down her face. "That could have been one of them.

"It's been nights I stayed up apologising and apologising to George Floyd for not doing more and not physically interacting and not saving his life," she said.

At the same time, "it's not what I should have done it's what he should have done," Ms Frazier added in a reference to Mr Chauvin, who was seated at the defense table.

Former Minneapolis police officer Derek Chauvin

Ms Frazier described Mr Floyd as "scared" and "terrified" and "begging for his life."

"It wasn't right. He was suffering. He was in pain," she said. "I knew it was wrong. We all knew it was wrong."

Another witness said he made an emergency 911 call right after watching Mr Floyd's death to report a "murder."

Donald Williams, 33, recounted how he pleaded with officers on the scene to render assistance to Mr Floyd, who was being arrested for allegedly passing a counterfeit $20 bill.

Mr Williams, a mixed martial arts instructor, said Mr Floyd was already in "danger" when he arrived on the scene.

"You could see that he was trying to gasp for air, trying to breathe," he said. 

"You could see his eyes slowly rolling back in his head."

Mr Williams said Mr Floyd was being held by Mr Chauvin in a "blood choke" used in wrestling or martial arts and he saw him lose consciousness.

After an unconscious Mr Floyd was taken away in an ambulance, Mr Williams said he made the 911 call.

"I believed I witnessed a murder," Mr Williams told the court. "I didn't know what else to do."

Portions of his 911 call were played in the courtroom.

"He just went and killed this guy," Mr Williams said in the call. "Murderers, bro... they just killed that man in front of the store."

Asked who he was referring to, Mr Williams said "the officer sitting over there" and pointed to Mr Chauvin.

"Did you see Mr Floyd fighting back?" prosecutor Matthew Frank asked Mr Williams.

"No," Mr Williams said.

Under questioning from Mr Chauvin's defense attorney, Eric Nelson, Mr Williams acknowledged that he called Mr Chauvin and the officers names at the scene. 

"You called him a tough guy," Mr Nelson said. "You called him a 'bum' 13 times."

"They were not listening to anything I was telling them," Mr Williams said. "I had to speak out for Floyd."

The video of Mr Floyd's death was played for the nine-woman, five-man jury yesterday and is expected to take centre stage at the trial.

Prosecutors are seeking to demonstrate that Mr Chauvin had no justification for using a dangerous hold on Mr Floyd that led to his death.

Mr Chauvin's attorney countered yesterday that Mr Floyd was on drugs and his death was caused by the drugs and a medical condition rather than asphyxiation.

"You will learn that Derek Chauvin did exactly what he had been trained to do," Mr Nelson said.

Mr Chauvin, a 19-year veteran of the police force, faces up to 40 years in prison if convicted of the most serious charge - second-degree murder.

The trial is drawing worldwide attention and the White House said yesterday that President Joe Biden will be among those "watching closely".

Ben Crump, an attorney for the Floyd family, called it a "landmark trial that will be a referendum on how far America has come in its quest for equality and justice for all."

The trial is expected to last about a month.

Three other former police officers involved in the arrest - Tou Thao, Thomas Lane, and J. Alexander Kueng - are to be tried separately later this year.