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Voters in both Western states approved ballot initiatives in 2012 allowing personal possession and use of marijuana by anyone aged 21 and or older for purposes of just getting high, though public consumption of pot remains illegal.
In January the world's first state-licensed retail marijuana outlets opened for business in Colorado, and stores in Washington are set to follow suit later this year. Both states are among 20 that have already removed criminal sanctions for medical use of marijuana.
The federal government still classifies marijuana as an illegal narcotic, but the Obama administration has given states new leeway to experiment with legalised cannabis.
In Denver's Civic Center Park near the state capital, revellers gathered to hear music and listened to speakers during a weekend event that organisers billed as the "world's largest 4/20 rally."
The date of April 20, or 4/20, corresponds to the numerical code widely recognised within the cannabis subculture as a symbol for all things marijuana.
Police officers standing by on the fringes of the Denver festival issued 63 citations, most for smoking pot in public - a ticket that carries a fine of $150. About half as many were cited on Saturday, police said.
At least eight individuals were taken to a detoxification facility for treatment during the two days, police said.
Denver police spokesman Sonny Jackson said officers have refrained from wading into the crowd to arrest violators, but instead were citing people who openly defied the public consumption ban.
"Those ticketed were blatantly in violation of state law and city ordinances," Jackson said.
Organisers of the rally and city officials beefed up security at the event after three people were wounded by gunfire at last year's rally.
Separately, the Cannabis Cup, a trade show sponsored by High Times magazine, drew sold-out crowds over the weekend at a Denver convention venue.
The two-day event featured marijuana sampling and workshops, such as how to open a pot shop, cultivation tips, and how to talk to children about weed, according to the event's website.
Rachel O'Bryan, spokeswoman for Smart Colorado, an organisation that advocates for stricter enforcement of marijuana laws, said the cannabis industry needs to do more to police its own.
"People are flouting the law by openly consuming," she said. "We're concerned about the message that sends to our kids."
In Seattle, several hundred people who paid $15 a head crowded the interior of a former brewery where Rainier Beer was made for decades to attend a 4/20 gathering organised by sponsors of the city's annual Hempfest rally.
Reggae music played over loudspeakers and the air inside was thick with the sweet, skunky odour of cannabis. But no police were visible at the event, which organisers deliberately held on private space leased from the brewery owners in an industrial section of the city south of downtown.
The gathering featured a workshop on how to roll a joint with an entire ounce (28 grams) of marijuana - the legal limit for personal possession in the state - as well as vendors selling pipes and other paraphernalia, and a blind-toke test in which participants tried to distinguish between different strains of pot by sampling them. Attendees ranged from middle-aged baby boomers to a younger crowd from the so-called millennial generation.
A cheer from the crowd went up at precisely 4:20 p.m. local time, as many attendees milling about outside lit joints and pipes simultaneously, sending puffs of smoke into the air followed by raucous fits of coughing.
One woman in the crowd accepted a joint handed her from a bearded bystander.
"You look just like Jesus," she exclaimed in an apparent reference to the event coinciding with Easter Sunday. "How does it feel to be risen?"
Doug Medina, 54, said he traveled hundreds of miles with his wife and daughter from Billings, Montana, to Seattle for the 4/20 weekend celebrations there.
"It feels a little more open than it did five or 10 years ago," he said while smoking a joint outside the brewery.