Cullars-Golden said she lost her son Marcus Golden at the hands of Saint Paul police last year -- shot in the head.
"I am so traumatized," she said of her son's death.
Like Philando Castile, the man fatally shot on Wednesday, Golden was black.
"I wanted to come out today to show my support because many of you have been supportive of my family," Cullars-Golden said, joining a lineup of activist speakers at the protest.
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Some in the crowd were angry, while others wiped away tears. The protesters of all ages and backgrounds stood shoulder-to-shoulder, chanting their demands for justice.
"Stop executing black people," read one sign. T-shirts bore slogans including "Hands Up, Don't Shoot," and the name of the national advocacy group, Black Lives Matter.
Castile was the second black man in two days fatally shot by US police. They are the latest in a string of similar cases that have fueled outrage across the United States, from city streets to the White House.
Amplifying the horror for many was the video that was live-streamed in the shooting's aftermath by the slain man's girlfriend, Diamond Reynolds, as she sat in the passenger seat. In the backseat was her 4-year-old daughter, Dae'Anna.
Castile, 32, had been pulled over Wednesday in Falcon Heights, near Minneapolis, for a broken tail light.
He was shot after informing the officer that he had a gun and a permit to carry, and then reaching for his wallet, according to Reynolds.
In the southern state of Louisiana on Tuesday, 37-year-old father of five Alton Sterling was pinned to the ground by police outside a convenience store and shot several times at point blank range.
Banks said Castile had worked in the cafeteria at her sons' elementary school.
She said she did not know how to break the news of the death to her boys.