One of four videos from the April 4 incident shows the officer lying on the back of 26-year-old Patrick Lyoya as the two scuffled after a traffic stop and then appearing to shoot him in the head.
Prior to the shooting, the men appear to be wrestling on the ground for control of the officer's taser. The officer has not been named.
TV footage showed some 50 to 100 protesters convened in downtown Grand Rapids, a city of around 200,000 people. They carried "Black Lives Matter" placards and chanted "no justice, no peace."
"It was just a progression of sadness for me. The loss of life in any circumstance is sad and I know that it's going to impact our city," he added.
Police killings of Black Americans have drawn acute national attention in recent years, particularly after a white Minneapolis police officer kneeled on the neck of George Floyd until he died in 2020.
Floyd's death, which was filmed by a bystander in a video that went viral, fueled the Black Lives Matter movement and sparked months of protests against racial injustice and police brutality in the United States and around the world.
The officer in the Grand Rapids case is on paid leave while state police investigate whether to bring charges, Winstrom said.