Ten people are dead and 15 are injured after a white van ploughed into a lunchtime crowd in northern Toronto near Yonge and Finch on Monday afternoon, said police at a press conference.
The driver is in custody and the investigation is ongoing, added Toronto Police Deputy Chief Peter Yuen.
Federal officials have said the attack does not represesent a larger threat to national security.
The suspect, who was quickly arrested, has been identified as Alek Minissian, 25. The authorities say he was not previously known to them.
Police still don't have a known cause or motive for the attack. But Euronews' partners NBC News have been briefed by US and Canadian law enforcement officials who say that mental illness is currently the leading theory based on their preliminary investigations.
Police say the van had been driven along pavements, ploughing into pedestrians.
"The actions definitely looked deliberate," Toronto Police Chief Mark Saunders told the late night news conference. The attack happened not far away in the northern section of Canada's biggest city.
Federal Public Safety Minister Ralph Goodale said there appeared to be no connection to terrorism.
"On the basis of all available information at the present time, there would appear to be no national security connection to this particular incident," he said.
Video has emerged apparently showing the scene as the suspect was arrested. Amid a tense standoff police challenge a man who appears to have a gun. An officer has been praised for not opening fire. The suspect was told to get down and he was detained without shots being fired.
Prime Minister Justin Trudeau has condemned what he called a "tragic and senseless attack".
Sunnybrook Health Sciences Centre received a total of 10 injured patients from the scene. Two people were pronounced dead upon arrival, five people are in critical condition, two in serious condition, and one in fair condition, according to a statement.
New York General Hospital received four patients: one is in critical condition and three are in stable conditions.
Minissian is reportedly due to appear in a Toronto court on Tuesday morning.