More than a dozen fans hoping to see Garth Brooks perform surprise Denver show treated for heat exhaustion

Some of the fans who lined up outside a Denver club Thursday hoping to see Garth Brooks perform there have experienced medical issues that are apparently due to heat. About 15 to 20 people were treated for heat exhaustion, according to Julie Browman, spokesperson for Adams County Fire Rescue.

The country music superstar may be performing at the Grizzly Rose on Thursday night in a surprise concert that has been advertised only as a "Yukon Jack" show. Brooks is playing concerts at Ball Arena on Friday and Saturday, plus two shows there on Sunday, as part of his Blame It All On My Roots Tour.

The temperatures in the Mile High City on Thursday rose into the low 90s between 2:30 p.m. and 3 p.m. The heat-related issues then started happening after fans who were there began gathering in a large crowd around 4 p.m., which was when The Grizzly Rose said people could begin lining up. First responders who were already on the scene began helping people after yells of "medic" came from the crowd.

First responders and staff from The Grizzly Rose were handing out water to some fans.

One fan named Annette said she arrived at 1 p.m. and was disappointed by the behavior of some people who were lined up.

"It's very hot and this was ridiculous. This -- with all the pushing and ... they need to pick something (else)," she said.

Doors at the venue weren't expected to open until the early evening but people were being let in earlier than that.