All police officers shall arrest any person found violating any state law or city ordinance, when such violation is committed in the presence of the officer; and when an offense has been committed in the presence of the officer; and when an offense has been committed, but not within the knowledge of the officer, and there is reasonable grounds to believe that the person accused has committed it, such officer shall forthwith cause proper complaint to be made, and cause a warrant to be issued for the arrest of the accused. It shall be the duty of policemen, and they shall have the power and authority to serve all summons and execute all processes placed in their hands for the arrest, apprehension, trial and commitment of any person charged with the violation of any ordinance of the city, and may serve writs in all matters pertaining to violations of the state laws.
(‘68 Code, § 3.07)