Penal Code § 410
TITLE 11. OF CRIMES AGAINST THE PUBLIC PEACE
If a magistrate or officer, having notice of an unlawful or riotous assembly, mentioned in this Chapter, neglects to proceed to the place of assembly, or as near thereto as he can with safety, and to exercise the authority with which he is invested for suppressing the same and arresting the offenders, he is guilty of a misdemeanor.