Mo. Rev. Stat. § 544.216
Except as otherwise provided in section 544.157, any sheriff or deputy sheriff, any member of the Missouri state highway patrol, and any county or municipal law enforcement officer in this state, except those officers of a political subdivision or municipality having a population of less than two thousand persons or which does not have at least four full-time nonelected peace officers unless such subdivision or municipality has elected to come under and is operating pursuant to the provisions of sections 590.100 to 590.150*, may arrest on view, and without a warrant, any person the officer sees violating or who such officer has reasonable grounds to believe has violated any ordinance or law of this state, including a misdemeanor or infraction, over which such officer has jurisdiction. Peace officers of a municipality shall have arrest powers, as described in this section, upon lands which are leased or owned by the municipality in an unincorporated area. Ordinances enacted by a municipality, owning or leasing lands outside its boundaries, may be enforced by peace officers of the municipality upon such owned or leased lands. The power of arrest authorized by this section is in addition to all other powers conferred upon law enforcement officers, and shall not be construed so as to limit or restrict any other power of a law enforcement officer.
§ 544.216, RSMo