The teenager whose film of George Floyd's death sparked global protests said she "stays up apologising" to him for "not doing more".
Darnella, now 18, was one of four young witnesses who to take the stand on the second day of Derek Chauvin's trial.
She told the court of seeing Mr Floyd "begging for his life," comparing him to her dad, brother, cousins and uncles "because they are all black".
Issues of racial equality and policing lay at the centre of the case.
The man accused of killing George Floyd
The jurors who will decide Derek Chauvin's fate
On Monday, the opening session of the trial heard Mr Chauvin, an ex-police officer, knelt on Mr Floyd's neck for over nine minutes while arresting him in Minneapolis in May 2020. Prosecutors say this was a "major cause" in his death.
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Defence lawyers have indicated they will argue that 46-year-old Mr Floyd died of an overdose. Mr Chauvin, 45, denies charges of murder and manslaughter.
Three other officers who were present - Tou Thao, J Alexander Keung and Thomas Lane - will go on trial later in the year.
What did the trial hear on the second day?
Four children who were all under 18 at the time of the incident gave evidence to the court, but the cameras were switched off so the jurors could not see them and they were identified only by their first names.
Darnella, then 17 years old, was walking to the Cup Foods shop with her nine-year-old cousin when they came across the arrest on the street outside.
She told the court she started filming on her phone because "I saw a man terrified, begging for his life. It wasn't right - he was in pain."
She described hearing Mr Floyd "saying 'I can't breathe'. He was terrified, he was calling for his mom."
Darnella said witnessing his death had changed her life.
"When I look at George Floyd I look at my dad, I look at my brother, my cousins, my uncles - because they are all black," she said, audibly crying. "And I look at how that could have been one of them."
"I stay up apologising to George Floyd for not doing more."
Her young cousin also gave evidence and said she felt "sad and kind of mad" by what she saw. "It sounded like he was hurting".
The last 30 minutes of George Floyd's life
Two friends, Alissa, 18, and Kalen, 17, had driven up to the store when they came across the arrest. Both described feeling helpless as they watched Mr Floyd's last moments before "he was just laying there, no longer fighting or resisting".
The last witness of the day was Genevieve Hansen, an off-duty firefighter, who said the officers prevented her from administering medical help that would have saved Mr Floyd's life. She was rebuked by the judge for her testy responses to defence questioning.