Thousands of marijuana enthusiasts gathered in the first two US states to legalise recreational cannabis for a celebration of weed culture which included rallies, concerts and trade shows.
Voters in both Colorado and Washington state voted 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 the UK small events were held to mark the day; dubbed 4/20 after the date of April 20, which corresponds to the numerical code widely recognized within the cannabis subculture as a symbol for all things marijuana.
Hundreds of people flouted drug laws in London and Manchester to openly smoke cannabis. Police officers were out in force at a meeting in London's Hyde Park to show their support for decriminalisation of the drug.
In the US 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 capitol, revelers on Sunday gathered to hear music and listened to speakers during a weekend event that organizers billed as the "world's largest 4/20 rally."
This month saw the state of Colorado sell recreational marijuana for the first time. The wildly successful first month of sales generated $14m from just 59 marijuana businesses. Crime in the city would also appear to be down since the legalisation move.
Police officers standing by on the fringes of the Denver festival yesterday issued 63 citations on Sunday, most for smoking pot in public - a ticket that carries a fine of $150. About half as many were cited on Saturday, 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.
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 organization 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."
Additional reporting by Reuters