Two men were stabbed, a 34-year old in the hand, an 18-year old in the back, late Tuesday night.
Their injuries are serious but their lives are not in danger, according to police.
Two suspects, both men in their twenties, are in custody.
"Investigators are trying to figure out how they are involved in the event," said Montreal police Constable Danny Richer.
Officials said onlookers threw projectiles at officers protecting the crime scene.
The brawl erupted in a strip mall parking lot around 11:30 p.m., after the Canada Day fireworks show across the street had ended. Investigators are not yet sure what triggered the fight.
A city of Montreal employee working at the festivities said he was shocked to hear about the stabbing because while there were huge crowds people had remained calm.
“I never saw so many people, the fair was good, the show was good, the fireworks were good. There were a lot of people but it was very calm, there were a lot of police officers and when we left around midnight there were almost no people left,” Charles Larivière said.