Chauvin, 45, is charged with murder and manslaughter, accused of killing Floyd last May by pinning the 46-year-old handcuffed Black man to the pavement for what prosecutors said was 9 minutes, 29 seconds. Floyd was arrested after being accused of trying to pass a counterfeit $20 bill at the convenience store.
Chauvin continued to kneel on Floyd while fellow Officer Tou Thao held the crowd of about 15 back, even when one of the onlookers identified herself as a firefighter and pleaded repeatedly to check Floyd's pulse, according to witnesses and bystander video.
The firefighter, Genevieve Hansen, wept on the witness stand as she recalled how she was not allowed to give any medical assistance or tell the police what to do, such as administering chest compressions.
"There was a man being killed," said Hansen, who testified in her dress uniform and detailed her emergency medical technician training. "I would have been able to provide medical attention to the best of my abilities. And this human was denied that right."
She said she felt "totally distressed" when she couldn't help.
She's seen on video pleading with officers to let her attend to Floyd and begging them to check his pulse.
VIDEO: Firefighter Genevieve Hansen takes the stand on day 2
Donald Williams, who watched Chauvin kneel on Floyd's neck, said he saw Floyd trying to gasp for air.
He teared up as he listened to a recording of the 911 call he made after emergency crews rushed Floyd to a hospital.
During the arrest, Williams, who is trained in mixed martial arts, warned Chauvin that he was using a potentially fatal "blood choke."
Williams testified that he called 911 after paramedics took Floyd away, "because I believed I witnessed a murder." In a recording of the emergency call, Williams could be overheard yelling at the officers: "Y'all is murderers, bro!"
On cross examination, Chauvin's attorney grilled Williams on his verbal assault of the officers, such as calling Chauvin "a bum" 13 times and using profanity.
VIDEO: Watch beginning of Darnella Frazier testimony from trial day 2
Also testifying Tuesday was Darnella Frazier, the woman whose cellphone video of Floyd's arrest sparked worldwide outrage. Surveillance video showed her first approach the scene with her 9-year-old cousin.
The judge didn't allow cameras to show Frazier's face in court because she was a juvenile at the time of the arrest.
She began crying when prosecutors showed her a photo of Chauvin at the time he had his knee of Floyd's neck. Frazier explained how the incident affected her.
"It's been nights I stayed up apologizing and apologizing to George Floyd for not doing more and not physically interacting and not saving his life," she said.
RELATED: Onlooker anger increased as George Floyd stopped moving, witnesses say
The Associated Press contributed to this report.