DENVER—Marijuana conference ads on Denver-area buses have law enforcement groups complaining they promote illegal drug use.

The ads for this weekend's KushCon "cannabis lifestyle" convention in Denver are believed to be the first marijuana-related advertising on public transit anywhere in the country. The ads say "Have a Kush Day!" and promote the convention. Kush is a type of marijuana and the name of the California-based sponsor, Kush Magazine.

The Colorado Association of Chiefs of Police and the Colorado Drug Investigators Association have complained about the ads to the Regional Transportation District.

The Denver Post reports that RTD will keep the ads, but modify them to better reflect that they are paid advertisements.