A crowd estimated at more than 10,000 people gathered outside the Vancouver Art Gallery Friday for the annual marijuana smoking event known as “4/20.”

Police blocked off streets all around the downtown locale for hours in the late afternoon, clogging rush hour traffic.

Similar gatherings were staged in cities across North America, as they are every April 20, often featuring speakers who address the crowds with calls for loosening or eliminating marijuana laws.

Despite a prominent police presence on the perimeter, many people at the Vancouver event openly smoke marijuana and few are ever arrested.

"At the 4/20 event, conducting enforcement on minor possession of marijuana is not our priority,” said police spokesman Lindsey Houghton.

Some Vancouver parents have expressed concern because the city’s school board scheduled Friday as a professional development day, meaning classes are cancelled at most schools, leaving many teenage students free to attend the 4/20 event.

"I don’t think it's very responsible," Inge Mueller-Langer, mother of two teens, told CBC News. "If we had an event where children were being offered tequila shots, we would all be in an uproar.”

Drug prevention experts like Tibor Palatinus, of Narconon, say 4/20 is a bad idea.

"It’s not a festival," Palatinus said. "It’s a pot promotion. It's a weed-selling promotion. Sponsored and promoted by drug dealers. That's what is going on."

The head of the Vancouver School Board’s drug prevention program, Art Steinmann, said that although there are no classes, the Vancouver School Board is co-hosting a free concert all afternoon at East Vancouver’s Rio Theatre in an attempt to discourage students from heading to the 4/20 event.

The 4/20 tradition has many reputed origins, but it's believed to have originated in the 1970s at a California high school where students would gather at 4:20 p.m. every weekday to smoke pot.

With files from the CBC's Natalie Clancy