The chief of police, in the execution of his powers and duties in arresting any person accused or suspected of crime, with or without a warrant, or in the suppression of any riot or unlawful assembly or breach of the peace, or in preventing the violation of any provision of this code, shall have the power to call to his aid any citizen of the city, and it shall be unlawful for any person so called to refuse to obey. (1969 Code)