Marijuana fans held mass rallies and pot parties in Canada and the United States, easing into the weekend with an annual celebration devoted to the feel-good weed.

Western Canadian metropolis Vancouver closed a major road in the financial district for the day-long demonstration, in which attendees are urged to light up, while the sweet-smelling smoke also wafted heavily over California.

Vancouver organizers and media said between 10,000 and 15,000 attendees made Vancouver's the biggest of the so-called 4/20 events. A crowd of mostly young adults spent the day lounging in sunshine listening to bands, many of them smoking weed or munching snacks sold by street vendors.

Near 4:20 pm, a woman dressed in a green tutu handed out free joints of marijuana, and the crowd sang a version of the national anthem that began with "Oh, Cannabis," instead of, "Oh, Canada."

Police officers, scattered in groups at the edge of Vancouver's crowd, told AFP there were no incidents, other than "a few, less than 10" people who felt ill after smoking and were taken away by ambulance.

In the Canadian capital of Ottawa, local media reported that 5,000 people attended a rally at Parliament Hill.

Called "420" after a nickname for marijuana -- hence the April 20 date -- the event has murky beginnings.

It reportedly began decades ago at a California high school where a group of students lit joints at 4:20 pm, when classes ended. The event has spread throughout North America and around the world.

While activists use the day to press for decriminalization or legalization of the drug, for others it's an excuse to party.

"Have a safe happy holiday my fellow stoners! I'm off to smoke a fattttttttttttttttttttty :)" posted the Facebook group 420, based in Vancouver and with more than 200,000 followers, referring to a sizable marijuana joint.

For more than a decade this Pacific port city has tackled illegal drugs as a health instead of a criminal issue, bucking North America's so-called War on Drugs.

Meanwhile, it has become a global center of the annual April 20 celebration of marijuana.

Vancouver's police priority is to stop traffickers of hard drugs like cocaine and methamphetamine, Constable Lindsey Houghton told AFP. At 420, "we typically do not have any concerns as long as they are peaceful," he said.

Most local activists for marijuana decriminalization or legalization say the herb should be regulated - and minors should not smoke pot.

Drug researcher Tim Stockwell, director of the Centre for Addictions Research of British Columbia at the University of Victoria, said much scientific research supports legalization, or at least decriminalization for health and social reasons.

In Australian, American and European jurisdictions where laws have been relaxed, the sky has not fallen," he noted.

"It's not a completely benign substance but on a scale of harm it rates below alcohol and tobacco as well as cocaine and heroin."

In former hippie haven California meanwhile the day was marked with parties, discount at so-called "medical marijuana" outlets and protest action by lobbyists seeking legalization.

"It's the holidays for people who love marijuana," said Dave Brian, editor of The 420 Times based in Burbank, just north of Los Angeles, which reported that 10,000 people gathered in the University of Colorado last year to smoke.

This year a Non-Governmental Organization called NORML, which promotes the legalization of marijuana, organized an event in Orange County, south of Los Angeles, as well as other private parties and promotional offers.

For example Kushmart, a dispensary in the heart of Hollywood, was offering 50 percent and more off of cannabis variety O.G. Kush, described as a third stronger than any other marijuana.

California is wrestling with how to clamp down on retailers -- who often call themselves medical marijuana dispensaries, citing the powerful pain-killing effect for cancer sufferers among others -- especially near schools.

"There are some people that want to keep it just medical and there are people who say actually recreational smoking is medical to begin with anyway," said Brian.

"They have a point," he added.

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