But it's not the pot smoking they're concerned about at the yearly event, billed as the nation's largest April 20 celebration. Instead, police say they're 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."
Organizers say the event -- which drew 50,000 people last year -- could bring a record 80,000 this year, since it's the first celebration since Colorado and Washington voted to make pot legal for recreational use.
Even with the legalization, Colorado law bans open and public marijuana use. Still, authorities generally look the other way. The smoke hangs thick over a park at the base of the state Capitol, and live music keeps the crowd entertained well past the moment of group smoking at 4:20 p.m.
Group smoke-outs are also planned today 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.
Denver's celebration this year also features the nation's first open-to-all Cannabis Cup, a marijuana competition patterned after one held in Amsterdam.
Similar to a beer or wine festival, pot growers compete for awards for taste, appearance and potency of their weed. Denver's event, sponsored by High Times magazine, has sold out more than 5,000 tickets. Snoop Lion, the new reggae- and marijuana-loving persona for the rapper better known as Snoop Dogg, will receive a "Lifetime Achievement Award" from High Times. And the hip-hop group Cypress Hill was set to perform a sold-out concert this evening in Colorado's iconic Red Rocks Amphitheatre.
The celebration should be especially buoyant this year, organizer Miguel Lopez said, because it marks the first observation since Colorado and Washington voted to defy federal drug law and declare pot OK for adults over 21.
Both states 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.
Lopez said the holiday is more than an excuse to get high -- it's also a political statement by people who want to see the end of marijuana prohibition.
"You don't have to smoke weed to go to 4/20 rallies. You don't have to be gay to go to a Pride festival. You don't have to be Mexican to celebrate Cinco de Mayo," Lopez said. "That's what this is. It's a celebration, it's a statement about justice and freedom and this movement."
Colorado's weekend celebrations drew plenty of marijuana activists from out of state.
"Never have I ever imagined I could do this on American soil," said Eddie Ramirez, an Austin, Texas, pot user who attended a "420 Happy Hour" Friday at a downtown Denver hotel. "Being a smoker my whole life, this has been on my bucket list -- go scuba diving, go deep-sea fishing and go to the Cannabis Cup."
One place pot-smoking won't be as evident this year is the University of Colorado in Boulder. The school once was home to the nation's largest group smoke-out on April 20. More than 10,000 people showed up in 2010, and in 2011 Playboy magazine cited the celebration and named the campus the nation's No. 1 party school.
Last year, school officials closed the site of the party, Norlin Quad, on April 20. They planned to rope off the area again this year.
Lopez conceded that many don't appreciate the April 20 smoke-outs. But he insisted they at least force marijuana critics to talk about the drug and consider its legal status.
"Not everybody likes everything in America. That's one of the great things, that we can express ourselves," Lopez said.
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