A clash between protestors and police erupted at an unsanctioned Cannabis Day protest outside the Vancouver Art Gallery Wednesday, resulting in multiple arrests and fines.

Const. Brian Montague said officers dealt with protest organizers and participants peacefully over the course of the day, but things came to a head when one man at the anti-prohibition rally allegedly refused to stop selling marijuana.

Police, who said they witnessed the man selling pot to young people, eventually moved in to arrest him.

“Officers were immediately confronted and swarmed,” Montague said in a release. “Police were required to pepper spray at least one person to complete the arrest.”

A loud, chaotic scene broke out that ended with dozens of protesters chanting as police handcuffed a total of four people and took them into custody.

Protester Joshua Helsdon witnessed the melee firsthand and was one of the people pepper sprayed.

“[The police] started ripping, tearing people apart and stepping on them,” he said.

Montague said Cannabis Day participants had been warned in advance that selling marijuana would not be tolerated. Authorities were concerned about a repeat of April’s 4/20 rally, where 64 of the roughly 25,000 attendants ended up in hospital with nausea and other symptoms, many of which were attributed to taking edible marijuana products.

No serious injuries were reported, however.

Cannabis Day organizers initially planned to erect a stage and have vendor booths similar to the ones at 4/20, but the City of Vancouver sent them a cease and desist letter warning them not to do so without the proper permit.

Organizers agreed, but protesters still turned up at the art gallery in large numbers, eventually blocking some busy downtown streets.

Police said one of the men arrested faces trafficking charges and the other three people face charges of obstruction.

The incident happened a week after Vancouver became the first city in the country to license and regulate medical marijuana dispensaries.

With a report from CTV Vancouver's Scott Roberts