One person has been shot in the leg and another suffered unknown injuries after shots were fired at a marijuana celebration held to mark the counterculture holiday 4/20.
Panic broke out in the crowd as the sound of gunfire rang out among the crowd of several thousand.
Witnesses said they heard three or more shots fired at the celebration just before 5pm local time.
Denver police rushed to the scene and crime tape was placed around a pavilion at the event.
Several thought firecrackers were being set off, then a man fell bleeding, his dog also shot.
Travis Craig, 28, who saw the shooting and said he used a belt to apply a tourniquet to the man's leg, said: "I saw him fall, grabbing his leg."
"He was just screaming that he was in pain, and wanted to know where his girlfriend was. She was OK. And then the cops showed up real quick, like, less than a minute. They put him on ambulance and left."
Aerial footage showed people running from the park.
The celebration on Saturday was the first since Colorado and Washington made marijuana legal for recreational use.
As many as 80,000 people were expected to attend.
A sizeable police force on motorbikes and horses watched over the event.
Officers at the festival did not arrest people for smoking marijuana in public, which is still illegal.
The term 4/20 is thought to refer to either an earlier police code for the offence of using marijuana or a day or time that became significant to the counterculture.