DENVER (AP) – 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. They don’t believe the ads belong on something that is paid for with tax dollars.

“It’s an inappropriate use of public transportation to advertise a conference that promotes the illegal use of marijuana,” said Daniel Brennan, Colorado Association of Chiefs of Police President.

RTD says they will keep the ads, but modify them to better reflect that they are paid advertisements.

Scott Reed of the RTD says only 33 buses out a more than 1,000 are displaying the ad for what he emphasizes is a legal event.

“We do not endorse, promote any of the products, services, or events that are advertised on our buses, just as other media outlets do the same,” Reed said.

RTD says it only made a few thousand dollars on the ads.