Incident commander Dennis Kato said police received phone calls from residents starting at 9 p.m. reporting groups of young people were roaming Hollywood and attacking people.
The crimes do not appear related to the George Zimmerman acquittal protests, the Los Angeles Times said.
The robbers knocked down tourists and grabbed their phones, Kato said, and took at least one cash register from a business.
There are no reports of weapons being used, the Times said.
Forty robbers were believed to take part in the incident, police said, and the members of the large group appeared to know each other.
More than 110 officers were brought in from the Los Angeles police and other departments to track down the attackers, but many remain at large, the Times said.