Protesters threw rocks at police in Salt Lake City after officers shot a black teenager who was attacking a man late on Saturday, authorities said.
Relatives of the teenager identified him as Abdi Mohamed, 17, and told the local Fox 13 television news channel he was in a coma at a hospital after being shot three times. Police said he was in critical condition.
The Salt Lake City Police Department said he was shot after officers in the downtown Rio Grande area saw two people attacking a male victim with metal objects and ordered the pair to drop their weapons.
"One of the males complied and dropped the weapon, the other continued to advance on the victim and was shot by officers," the department said in a statement on Sunday.
The shooting came as police departments across the United States face increased scrutiny over allegations of excessive force against black people and other minority groups. High-profile killings of unarmed African-American men by police led to national protests and spawned the "Black Lives Matter" civil rights movement.
One man who said he was a witness, Selam Mohammad, 19, told reporters his friend, the teenager who was shot, was holding a broomstick and had been fighting with an older man before the officers arrived.
"He barely even turned around. ... My friend didn't do anything," Mohammad told the Deseret News newspaper.
Scores of police, some of them carrying riot shields, responded to the scene where crowds of angry onlookers hurled abuse, rocks and bottles at the officers. Authorities said four people were arrested for civil disorder.
The department said two officers were involved in the shooting and that both had been placed on routine administrative leave pending the investigation.
It said both officers were wearing body cameras, but that the footage was not being released yet because of the probe led by the Unified Police Department.
Fox 13 quoted relatives as saying Mohamed moved to Utah from Kenya about 10 years ago and lived in West Valley City with his girlfriend and their son.
His girlfriend, Becca Monson, told the channel he was a "really caring, good, loving" boyfriend and father.
Salt Lake City Mayor Jackie Biskupski said in a statement that while the investigation continued, "there is no doubt what happened is a tragedy for all involved and for our entire city."
(Reporting by Daniel Wallis in Denver; Editing by Frank McGurty, Ros Russell and Peter Cooney)