"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 some guys playing."
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.