Eight teenage girls have been charged with murder in the Saturday night stabbing of a 59-year-old man in downtown Toronto, authorities said.

Police said the girls, aged between 13 and 16, seem to have met online before meeting in person on the night of the attack, possibly for the first time.

They were arrested near the crime scene shortly after midnight on Sunday.

The man, who has not been named, had been living in a shelter for the homeless at the time of the assault.

"He does have a very supportive family in the area so I wouldn't necessarily call him homeless, maybe just recently on some hard luck," Toronto Police Detective Sergeant Terry Browne told reporters.

Officer Browne said the girls are believed to have assaulted and stabbed the victim in Toronto's downtown core, an area filled with high-end condominium towers and hotels, following an altercation. Police believe the man may have been preyed upon because he was spotted carrying alcohol.

A group of bystanders flagged down emergency services after finding the man with stab wounds, Mr Browne said. The man was rushed to a nearby hospital with serious injuries and died shortly after. Police recovered several weapons, but did not say exactly which type.

Police said the teenagers "swarmed" the man, adding the attack was a prolonged "back and forth" on the victim involving all eight of the teenagers.

"The actual incident, proper, lasted almost three minutes long," Mr Browne said. "So, walked away, walked toward, walked away, walked toward."

The attack came just hours before a mass shooting in Vaughn, Ontario, a small city just outside of Toronto. The sudden violence has unnerved some in Toronto ahead of the holidays.

Police have not called the teenage girls a gang. The "anomaly" of this attack, Detective Browne said, is that the girls all came from different parts of the city and did not appear to have met in person prior to the attack. Three of the girls had prior run-ins with police.

"We don't know how or why they met on that evening," he said.

A female resident of a nearby homeless shelter told the CBC the victim was stabbed in the stomach after trying to protect her when the girls approached her for alcohol.

"I didn't know if they had a knife or what. I was just scared," the woman said, explaining how she walked away from the attackers and sought refuge in the shelter.

In a statement, Toronto Mayor John Tory said he was "deeply disturbed" by the case.

The accused will now spend Christmas in jail as they await a 29 December court appearance. None of the suspects can be identified under Canada's Youth Criminal Justice Act but police said three of the suspects are 13 years old, another three are 14, and two are 16.

Investigators believe the girls may have got into other fights on Saturday evening and have appealed to the public for more information.