Two people were killed and at least 19 injured in a shooting at an outdoor party in Toronto, police said on Tuesday, raising fresh fears of a rise in gun crime in Canada's largest city.

Police said it was too early to say what prompted the shooting late on Monday in which a young man and young woman were killed. Many of those hurt were injured in the panic that ensued after shots were fired.

"This is the most serious crime of its kind to ever take place in the city of Toronto," Toronto Police Chief Bill Blair told reporters at the scene.

Coming barely six weeks after a shooting at a shopping mall that killed one and wounded six, it is likely to raise fears about the use of guns in a city which takes pride in its relatively low crime rate compared with U.S. urban centres.

Police said they had one person in custody but that more than one shooter was involved.

At the time of last shooting in Toronto's Eaton Centre the city's mayor, Rob Ford, described it as an isolated incident in "the safest city in the world".