There were high hopes that the nightly protests in Ferguson would end peacefully for a change Tuesday. That promise held out for most of the evening - until a water bottle flew at police.
By that time, most protesters had cleared out. Journalists outnumbered those who remained.
Officers put on helmets and shields, lined up on in front of some businesses and demanded a small crowd there clear out.
But when the bottle flew, officers broke into a sprint, chasing after young men.
This prompted a handful of agitated protesters to toss more bottles, glass and plastic.
Missouri State Highway Patrol Capt. Ron Johnson, who's tasked with maintaining peace, told reporters that "criminals" had thrown urine on police.
After the chase, number of riot police ballooned. Officers brought out K-9 dogs, and one policeman used pepper spray on some in the crowd.
Protesters locked hands in front of a police line, and some urged the crowd to remain peaceful.
Johnson credited them for preventing further escalation.
Police arrested 47 people, Johnson said, including a car of people driving by, who were armed.
"We identified a vehicle that made threats to shoot an officer," he said.
See more on this developing story on CNN.com