Any person or persons who in any public place or on any private property open to the public unreasonably or unnecessarily obstructs the free passage of other persons to and from or within such place or property and who shall fail or refuse to cease such obstruction or move on when requested to do so by the owner or lessee or agent or employee of such owner or lessee or by a duly authorized law-enforcement officer shall be guilty of a Class 1 misdemeanor. Nothing in this section shall be construed to prohibit lawful picketing.