"This is the worst incident of gun violence anywhere in North America. It's very shocking. A lot of innocent people were injured tonight," Toronto Police Chief Bill Blair said at the scene in Scarborough.
"Tonight's event is unprecedented. It's going to be shocking to everyone in Toronto, to everyone in Canada."
A teenage girl and a man believed to be in his early 20s, were pronounced dead at the scene, Blair said. Other victims suffered injuries from trampling.
A suspect was hospitalised and is in police custody, he said, adding police are still looking for other suspects.
Hundreds of young people were outdoors at the barbecue outside 193 Danzig St when gunfire broke out just before 11 pm yesterday, thestar.Com reported.
Police say the 21 victims, one a toddler under 3, were rushed to emergency rooms across the city.