The incident led to reports that the driver stopped a block away, got out with a rifle, and ran into a house, prompting rumors among the crowd now on edge that the person would return, according to Oregon Public Broadcasting.
KATU reported that police said the driver spoke to officers and cooperated and was released without charges.
That night, protesters tried to access the Portland Police Association office and threw objects at officers, eventually breaking into the building and setting a fire.
The night was also marked by two separate incidents of shots being fired in the crowd of protesters.
Portland Mayor Ted Wheeler, who has largely supported the protests, changed his tune Thursday.