Four people were arrested after a man was caught allegedly selling drugs to young people during an annual Cannabis Day protest at the Vancouver Art Gallery on Wednesday.

CBC reporter Jesse Johnston reported from the scene that police were wandering through the protest taking video of people openly selling marijuana.

Police swooped in on a pot vendor, and officers formed a human chain around the man while police put handcuffs on him.

Demonstrators came running over trying to get inside the wall formed by police, said Johnston. Police took several people away in handcuffs. One man claimed police had pepper-sprayed him.

Vancouver police and protesters face off Wednesday at the Cannabis Day protest in downtown Vancouver. (Tina Lovgreen/CBC)

Protesters briefly took over Howe Street chanting, "Let them go," but quickly dispersed.

Meanwhile, pot activist Marc Emery took to Twitter to ask more people to join them.

Come down to the Art Gallery and smoke up in support of injured, attacked, pepper-sprayed activists and by-standers assaulted by VPD. —@MarcScottEmery

Vancouver police later confirmed in a statement that four people had been arrested.

It said protesters had been warned that due to hospitalizations at previous Cannabis Day and 4/20 events, it would not tolerate the sale of marijuana products to young people.

Despite this, police said a man started selling pot to a youth and refused to stop.

Vancouver police confront protesters at a Cannabis Day protest at the Vancouver Art Gallery after police moved in to arrest a man allegedly selling pot to minors. (Jesse Johnston/CBC)

Police also confirmed they pepper-sprayed a second man when they were "swarmed."

They said one man faces potential trafficking charges, and another faces a potential obstruction of justice charge. Charges are expected against the other two people arrested as well.

Earlier this month, the City of Vancouver said the Cannabis Day event had grown to festival-size proportions with vendors and concessions, and was effectively a festival operating without a licence.

The city said it had grown far beyond its roots as a simple protest and ordered it to cease and desist.

Emery vowed the event would go ahead anyway, and said it could still be held as a protest without the concessions.