"I saw him fall, grabbing his leg," said Travis Craig, 28, who was at the celebration, saw the shooting and said he used a belt to apply a tourniquet to the man's 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."
The annual pot celebration this year was expected to draw as many as 80,000 people after recent laws in Colorado and Washington made marijuana legal for recreational use.
A sizable police force on motorcycles and horses had been watching the celebration since its start earlier Saturday. But authorities, who generally look the other way at public pot smoking here on April 20, didn't arrest people for smoking in public, which is still illegal.
Police said earlier in the week that they were focused on crowd security in light of attacks that killed three at the finish line of the Boston Marathon.
"We're aware of the events in Boston," said Denver police spokesman Aaron Kafer, who declined to give specifics about security measures being taken. "Our message to the public is that, if you see something, say something."
Stephanie Riedel, who traveled to the pot celebration from Pittsburgh, said she was dancing with a hula hoop when she heard pops. A man ran past her, then she said the crowd started screaming and running away. She was about 20 feet from the shooting and heard four or five shots.
"I couldn't make sense of what it was at first," she said. "We were all having a good time and I was in the mindframe of, we're here at a peace gathering. I thought it was some guys playing."
Rapper Lil' Flip was performing when the shootings occurred.
Aerial footage showed the massive crowd frantically running from the park.
Nationwide, group smoke-outs were planned for Saturday from New York to San Francisco. The origins of the number "420" as a code for pot are murky, but the drug's users have for decades marked the date 4/20 as a day to use pot together.