Reggae music filled the air, and so did the smell of marijuana, as celebrants gathered by mid-morning in a park just beside the state Capitol. Few police were in sight, though public marijuana use is still prohibited here.
"People aren't afraid to come out and celebrate it, the way they used to be," said 52-year-old Charles Bade of Denver.
Bade carried a tie-dyed flag with a big cannabis leaf and wore a garland of fake marijuana leaves around his neck. He said he's been marking 4/20 privately since he was 11 but decided to join this year's public smoke-out to celebrate the votes by Colorado and Washington last year to defy federal drug law and make pot legal.
Authorities generally look the other way at public pot smoking here on April 20. Police said this week 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.
Group smoke-outs are also 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.
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, received a "Lifetime Achievement Award" from High Times. And the hip-hop group Cypress Hill was set to perform a sold-out concert Saturday evening in Colorado's iconic Red Rocks Amphitheatre.
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 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."
"Does the behavior of the participants in these events reflect well on our state?" asked the head of Smart Colorado, Henny Lasley.
The organizer of Denver's 4/20 rally, Miguel 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."