Angry crowds hurled objects at police who retaliated in a second night of clashes in northern California following the death of a black man who was put in a chokehold by a New York police officer.
Police fired gas after being targeted by what they called "explosives". They moved in to clear roadways as protesters swarmed freeway overpasses at two locations in Berkeley, a Reuters reporter saw.
Police made several arrests among the demonstrators who numbered well over 500, and a number of stores in Berkeley were looted. One demonstrator who tried to prevent the looting was assaulted, police said.
Reuters could not immediately obtain further information from police, who some demonstrators said on Twitter had fired rubber bullets. Protesters have demonstrated daily in several U.S. cities since a grand jury's decision on Wednesday not to bring criminal charges against the white police officer whose chokehold contributed to Eric Garner's death in New York City in July.
The killings of Garner and of Michael Brown, an unarmed black teen, in Ferguson, Missouri, have highlighted the strained relations between police and the black community and rekindled a national debate over U.S. race relations.
New York was quieter over the weekend, but West Coast cities had braced for trouble after clashes in Berkeley and Seattle, Washington on Saturday.