(a) Any law-enforcement officer or public official responsible for keeping the peace may issue a command to disperse in accordance with this section if he reasonably believes that a riot, or disorderly conduct by an assemblage of three or more persons, is occurring. The command to disperse shall be given in a manner reasonably calculated to be communicated to the assemblage.
(b) Any person who fails to comply with a lawful command to disperse is guilty of a Class 2 misdemeanor.
(c) If any person remains at the scene of any riot, or disorderly conduct by an assemblage of three or more persons, following a command to disperse and after a reasonable time for dispersal has elapsed, it is prima facie evidence that the person so remaining is willfully engaging in the riot or disorderly conduct, as the case may be.