CALGARY -- Police say they were forced to bring in extra officers to a pot rally for the arrest of a marijuana activist because the crowd had become agitated.

"The crowd became aware of that occurring, then a number of people came out and it ... elevated the incident somewhat as people were becoming upset," Calgary police Insp. Mike Bossley said Thursday after Dana Larsen was charged.

"That really required additional response from the police just to make sure that it stayed as a peaceful event."

Larsen, who is from Vancouver, was in Calgary on Wednesday night with his "Overgrow Canada" campaign in which he aims to distribute one million pot seeds to be planted in public places.

He led an unsuccessful bid for a marijuana referendum in British Columbia and has run a medicinal cannabis dispensary for seven years.

After the rally, as angry supporters watched, officers escorted Larsen and another man to a police car. Supporters chanted "Shame! Shame!"

Police said a search of the van belonging to Larsen resulted in the seizure of 119 grams of marijuana, nearly 1,100 grams of marijuana seeds and a small amount of cannabis resin and oil.

Larsen, 44, is charged with one count of trafficking marijuana and a charge of possession for the purpose of trafficking. He was released from custody and is to appear in court May 18.

The other man was released without charges.

Bossley, who acknowledged that Larsen made it pretty clear what he was intending to do, said five officers were initially brought in for the rally. There were about a dozen on hand by the time of the arrest.

He said he doesn't think it was an over-reaction.

"The officers are there to keep the peace and to protect the public. We took what we believed to be the appropriate reaction with the limited number of resources present initially.

"As things became a little more heated, we ended up bringing additional officers there to support the event."

Prime Minister Justin Trudeau campaigned on a promise to legalize marijuana but has recently said that may be more difficult to do than expected. He wants to consult with the provinces before taking any action.

Larsen served 10 years as editor of Cannabis Culture Magazine and was a founding member of the B.C. Marijuana Party and the Canadian Marijuana Party.

In 2011, he ran for the leadership of the provincial NDP in B.C.