One man was arrested and taken to hospital after police moved in on Monday evening and quickly found themselves encircled.
There are two versions of what happened next: protesters say officers got heavy-handed, but a police spokesman says they were facing an unruly crowd.
"Officers were in the process of taking a male into custody when they were swarmed," Patrol Sgt. Wally Antoniuk said Tuesday.
"As a result, a Winnipeg Police Service member was physically assaulted. The male police were arresting escaped custody as a result of the assault."
The encounter was partially captured on video by a protester who streamed the event online. Police are seen asking the protesters, who totalled a few dozen, who their leader was. They also ordered them to get off Winnipeg's busy Portage Avenue and onto a sidewalk.
As officers started taking one man near the head of the parade into custody, other protesters stood around them. It was then that one officer was "pushed and shoved," said Antoniuk. What happened is not clearly shown on the video.
Police are then seen turning their attention to the man accused of shoving the officer. As they lead him to a police cruiser, he goes limp. The crowd begins chanting, "Shame on you" at the officers.
"A lady near me shouted, 'Oh my God. They Tasered him' or something to that effect," recalled Ken Harasym, the man who videotaped the event.
"Now I never saw a Taser. I didn't see anything that would lead me to that conclusion, but he was having ... what looked to be a seizure on the ground and the crowd went a little excited."
Police said the man suffered some sort of distress and no officers used a Taser. Harold Shuster, one of the protest organizers, said the man suffered a panic attack.
"He had an anxiety attack and that was the cause of the convulsions," Shuster said.
The video footage shows the crowd becoming more anxious and demanding that police call an ambulance. Officers had called for one immediately, Antoniuk said, and the man was taken to hospital and released.
Police said the man, a 20-year-old who they would not name, will face charges of assaulting a peace officer and obstruction.
After the suspect was led away, the march continued without any more confrontations. Protesters said the entire event would have been peaceful had police not intervened.
"I'm trying to understand ... why they chose to become immediately belligerent and try to force us off the street," Shuster said.
Both sides agree the protesters did not have a permit to stage the march down Portage Avenue, one of the city's busiest roads, and police were trying to get them out of traffic.
"I suppose by the letter of the law (the protest) was illegal because you do need a permit, but there have been two other situations where ... people have taken to the street without a permit and where the police have become involved, they have chosen ... the wiser choice, the mediating role," Shuster said.