A reckless exchange of gunfire at a crowded outdoor party in Toronto killed two young people in the city's east end.

At least 19 others, including an infant, were injured after bullets from more than one gun were sprayed into the crowd of more than 200 people.

"It is the most serious crime of its kind that has ever taken place in the city of Toronto," said police chief Bill Blair.

Police appealed for witnesses to contact them as they began to pull together descriptions for what appeared to be multiple shooters..

Officers said they had one "person of interest" in custody and were looking for other suspects.

The incident took place before 11pm at the outdoor barbecue.

"An altercation broke out among some individuals and there was an exchange of gunfire. During that gunfire a number of innocent bystanders were struck," said Mr Blair. "It's a very shocking event, a lot of people were injured tonight."

The two killed in the shooting were a teenage girl and a man in his 20s, officials said.

Mr Blair did not provide the age of the child who was hurt but said the injuries are "not life-threatening and the child will be fine".

Those hurt in the incident suffered a mix of gunshot wounds and injuries from the mass panic that ensued.