Gunfire erupted at an annual US marijuana celebration, injuring two people and scattering a crowd of thousands after they had just marked the first 4/20 counterculture holiday since Colorado legalized marijuana.
The man and woman who were shot were expected to survive, and police were looking for one or two suspects, said Denver Police spokesman Sonny Jackson.
Police asked festival attendees for possible photo or video of the shootings, and they had no immediate motive.
Witnesses described a scene of panic. Several thought firecrackers were being set off, then a man fell bleeding, his dog also shot.
"I saw him fall, grabbing his leg," said Travis Craig, 28, who 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 celebration this year was expected to draw as many as 80,000 people after Colorado and Washington last year voted to make marijuana legal for recreational use.
Aerial footage showed the massive crowd frantically running from the park.
A sizable police force had been watching the celebration, but authorities, who generally look the other way at public pot smoking 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 after Monday's attacks that killed three near the finish line of the Boston Marathon.
Stephanie Riedel said she was dancing with a hula hoop when she heard pops. A man ran past her, and she said the crowd started screaming and running away. She was about 20 feet (6 meters) 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 some guys playing."
Ian Bay, who was skateboarding through the park when shots erupted, said he was listening to music on his headphones when he looked to his right and saw 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.
Nationwide, 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 on legalization 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 legalization, 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."