It is unlawful for three or more persons to assemble together in a violent manner, to the disturbance of others, and with any use of unlawful force or violence by them or any of them against another person or causing property damage. No person shall willingly join in or remain part of a riot, knowing or having reasonable grounds to believe that it is such.
(Iowa Code § 723.1) (1999 Code, § 40.05) Penalty, see § 10.99