We've done demos for schools and also the police and the military. There are people there who would like to challenge you and heckle you but its normal.
Practice your choreography and techniques. Its always nice to see some fast techniques and some hardfalls. The hecklers usually become quiet when they see people getting thrown. Or maybe not, but the reaction would be like "OMG!, holy$h!#, how'd he do that?!"
I posted this video a while back, maybe this'll help.