Police say a mob of young people smashed windows and threw rocks and bottles in San Bernardino, resulting in at least six arrests.
Police are trying to determine whether Thursday night's civil unrest was arranged through social media contacts and whether it was sparked by the acquittal of George Zimmerman in the shooting death of Trayvon Martin. Police say some youths indicated there was no connection.
Lt. Mike Madden says about 100 people threw rocks and bottles at cars and at arriving officers. When the group was ordered to disperse, some took off running and began attacking and robbing pedestrians.
Madden says two were held on suspicion of robbery.
On Wednesday night, a similar mob robbed people in Hollywood. More than a dozen arrests were made.
'Bash Mobs' in Long Beach
Meanwhile, police in Long Beach issued a warning Friday about a planned, sudden assembly of individuals who may attack people and businesses.
Police said more than 100 people took part in a "Bash Mob" July 9, committing thefts, property damage and assaults in downtown Long Beach.
"The Long Beach Police Department takes this issue very seriously and will be prepared to arrest anyone whose intent is to commit criminal acts," said Long Beach Police Department Sgt. Aaron Eaton.