Toronto police say none of the injuries were life-threatening. Two firearms were recovered and three people were in custody.
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The shooting took place as the Raptors were celebrating the team's first-ever basketball title, following last week's finals win over the Golden State Warriors.
The shooting triggered a chaotic scene at the edge of the crowd as people ran from the gunfire, but the event itself continued. Several minor injuries were reported in the stampede.
During a speech from one of the team owners, the host of the rally interrupted the proceedings to alert the crowd to an emergency and asked for calm.
Canadian Prime Minister Justin Trudeau, Toronto Mayor John Tory, NBA Finals MVP Kawhi Leonard and other players were among those on stage at the time.
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"I want to make sure everyone stays calm," said the host, sportscaster Matt Devlin. "This is serious. Everyone stay calm ... There is an emergency being dealt with."
Those on stage remained in place and speeches resumed shortly after.
Thousands celebrate Toronto Raptors at victory parade
Roughly 1.5 million people attended the downtown parade earlier in the day. The shooting happened at the later rally near the City Hall square, where tens of thousands of people had gathered.
The Associated Press and ABC News contributed to this report.