Andishae Akhavan has insisted he was not resisting arrest, and said there was no reason for the officer to punch him in the face.
But union president Tom Stamatakis disagreed, saying officers had observed Akhavan riding his bicycle in a manner that "put a lot of people in some risk, whether it's accident or injury."
"This was not just a person minding his own business on his way home, abiding by the rules of the road and complying with all the traffic-control devices," Stamatakis told CBC News.
The officers "did exactly what the public would expect them to do. They took enforcement action and this person made a number of decisions that I would argue probably escalated the situation beyond where it needed to go."
Akhavan was "allegedly confrontational with police" after being stopped at Robson and Beatty streets for allegedly running a red light, police said.
Akhavan said he was riding his bike without a helmet through Yaletown at about 10:45 p.m. PT when two officers stopped him. Akhavan said he got off his bike and started questioning the officers as one began writing him a ticket.
"At that point they decided to put cuffs on me," he said. "I was just speaking to them. I didn't raise my voice or anything, and they started putting the cuffs on me."
He says that's when the video showing Akhavan standing at the side of the road being restrained by two plainclothes officers starts.
Captured on camera
"What is this for?" he asks.
Akhavan's left arm moves as he turns his body to speak to one of the officers again.
"Relax," the officer orders.
Then Akhavan appears to get punched in the jaw.
"Relax your arm," the officer then says.
The incident was filmed by Mike Schwarz, who says he happened to see his friend surrounded by the officers as he drove by.
"I pull out my phone and literally I arrive on the scene within five seconds and hit record. That's what you see on Facebook," Schwarz told CBC News.
Akhavan did nothing before the camera started recording video to provoke such a reaction, Schwarz said.
"At first I was in complete shock, I couldn't believe it happened," he said. "I wanted to get involved or do something but I realized I wasn't going to help the situation."
Schwarz told the officers the incident would go on TV. He posted it to Facebook, and in just a few hours the video received thousands of comments and shares.
Akhavan was treated at the scene by Vancouver Fire and Rescue Services for a cut lip and released, police said he could still face charges.
The incident was reported by the officer to his supervisor shortly afterwards. It was also reported to the professional standards section of the Vancouver Police Department, Stamatakis said.
An investigation will be carried out by the Police Complaints Commissioner to determine whether the force used during the arrest was appropriate.
B.C. Almanac live chat
B.C. Almanac hosted a live chat on this story today. Click below to see a replay of what was a very busy and lively chat.