(20 ILCS 1815/40) (from Ch. 129, par. 268)
    Sec. 40.
    Whenever 12 or more persons, any of them armed with clubs or dangerous weapons, or 30 or more, armed or unarmed are unlawfully, riotously or tumultuously assembled in any city, village, or town, it shall be the duty of each of the municipal police officers, and of the sheriff of the county and his deputies, and of the commanding officer of such unit of the Illinois State Guard as may be present on duty, or any or either of them, to go among the persons so assembled, or as near them as safety will permit, and in the name of the State command them immediately to disperse; and if they do not obey, every person refusing to disperse shall be deemed guilty of a Class A misdemeanor of unlawful assembly;
and each such officer
having notice of such unlawful assembly and refusing or neglecting to do
his duty in relation thereto, as aforesaid, shall be guilty of a petty
offense.
    When persons so unlawfully assembled neglect or refuse, on command, to disperse, it shall be the duty of the municipal, township or county authorities, or military authorities, or either of them, to forthwith suppress such assembly and disperse the persons composing it, in such manner as may be most expedient.
    If in the efforts made to suppress such assembly, and to arrest and secure the persons composing it who neglect or refuse to disperse, though the number remaining be less than 12, any such persons, or any persons, present as spectators or otherwise, are killed or wounded, the peace officers, military officers or enlisted men, and persons acting with them or by their order, each and all of them, shall be held guiltless of any crime, and justified in law.
(Source: P.A. 77-2830.)