The shooting in the Canadian city sparked a panic among those partying near Nathan Phillips Square, where a reported millions of people had gathered in and around to mark the first franchise victory for the Canadian team.
Three suspects, and two firearms, were in police custody as Toronto police began their investigation, according to a tweet from the Toronto Police Department.
Video and images of the scene showed fans dressed in red fleeing, without a clear indicator of what exactly was happening.
According to Canada's Global News, Toronto police officers were called to the scene just before 4pm local time.
A tweet from Toronto police stated that four victims had been identified, two of which had serious, but not life-threatening injuries.
The Toronto Raptors did not immediately respond to the reports, but video posted onto the team's Twitter account showed the crowds packed with supporters on a bright day with clear skies.
The Raptors won their first franchise NBA championship in history late last week, with a 114-110 victory over the Golden State Warriors in Game 6 of the NBA Finals.
The Toronto Police Department did not immediately identify a motive behind the shooting in Toronto.