Montreal police have launched an internal investigation after a video surfaced online that appeared to show an officer hitting and shoving students at a protest.

In a YouTube video taken at a recent protest against tuition hikes, an officer is seen striking one of the protesters holding a large banner with a baton. Later, the officer appears to shove a girl to the ground.

Student activists allege police used excessive force and pepper spray at what was an otherwise peaceful protest. But police say there was a small group of troublemakers who were getting aggressive.

"As we can see in the video there was absolutely no provocation from the protesters. It was a totally impulsive gesture from the policeman," Quebec student group spokesperson Gabriel Nadeau-Dubois told CTV Montreal.

Montreal police Sgt. Ian Lafreniere said the demonstration was largely peaceful, but a small group "decided to surround a few police vehicles from the intervention squad, so that's the reason why you see police officers pushing a line."

"That's not a justification for what happened after that," Lafreniere said, and "a lot of questions remain."

"And that's the reason why the chief asked for a complete, full investigation," he said.

Nadeau-Dubois disputed the claim that police officers were surrounded at the protest.

"As always, we think that an inquiry on the police made by the police is not the best way to really find out what happened that day, to really put the light on those unfortunate events," he said.

About 200 students attended the protest last week in front of a Ministry of Education building.

With a report from CTV Montreal's Maya Johnson