Rapper Lil' Flip was performing when the shootings occurred.
Aerial footage showed the massive crowd frantically running from the park.
Ian Bay, who was skateboarding through Civic Center Park when shots erupted, said he was listening to music on his headphones when he looked to his right and saw a swarm of hundreds of people running at him.
"I sort of panicked. I thought I was going through an anxiety thing because so many people were coming after me," he said.
Before the shooting, reggae music filled the air, and so did the smell of marijuana, as celebrants gathered by mid-morning in the park just beside the state Capitol.
Group smoke-outs were planned 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.
Colorado and Washington are still waiting for a federal response to the votes and are working on setting up commercial pot sales, which are still limited to people with certain medical conditions. In the meantime, pot users are free to share and use the drug in small amounts.
A citizen advocacy group that opposes marijuana proliferation, Smart Colorado, warned in a statement that public 4/20 celebrations "send a clear message to the rest of the nation and the world about what Colorado looks like."
"Does the behavior of the participants in these events reflect well on our state?" asked the head of Smart Colorado, Henny Lasley.
A smaller Sunday event scheduled at the park was canceled.
Saturday's attack recalled a similar shooting that left a police officer dead at a crowded jazz concert in Denver's City Park last summer.
The 22-year-old suspect in that case pleaded guilty to second-degree murder and faces at least 16 years in prison when he is sentenced at a hearing scheduled for June 21.
His attorneys said he was being pursued by gang members when he drew his weapon and fired.
Associated Press writer Catherine Tsai contributed to this report.
Kristen Wyatt can be reached at http://www.twitter.com/APkristenwyatt