Four people were injured during a crowded victory rally for the Toronto Raptors basketball team in Toronto.
Thousands of sports fans were taken to downtown Nathan Phillips Square when shots were fired. Many scurried for cover.
The event was interrupted only briefly as officials calmed the crowd.
Three people were arrested. The authorities have asked the public to send footage to help with their investigation.
The police tweeted that none of the victims had life-threatening injuries although two were seriously injured.
They also said they had recovered two firearms from the scene. Toronto celebrates historic Raptors NBA victory
Up to two million fans gathered Monday in downtown Toronto for a parade in honor of the basketball team.
The Raptors, the only Canadian franchise company of the National Basketball Association, won last Thursday's final of the championship against the Golden State Warriors – the first Canadian team to win the American title.
By the afternoon, thousands of night owls gathered to watch the closing ceremony of the event with players and dignitaries, including Prime Minister Justin Trudeau.
Mr. Trudeau tweeted later, hoping for a "speedy recovery" for the injured.
"We will not allow this act of violence to escape the spirit of today's parade."
The mayor of Toronto, John Tory, said he was "disappointed" that the event was tarnished by force of arms.
Canadian sports fans were delighted with the Toronto Raptors, the team's first championship win in its 24-year history.
The team's players were promoted in five double-decker buses on the parade route, where they were welcomed as followers in Raptors jerseys and shirts. The crowd dispersed in the late afternoon without further incident.
Mr. Tory proclaimed this July 17 "We the North Day" to commemorate the historic season.