A man drove a white rental van into a crowd of pedestrians in the center of Canada's biggest city Toronto on Monday, injuring as many as 10 people, police said.
Canadian media reported at least four fatalities, but that was not yet confirmed by police.
The vehicle fled the scene, but the driver was later arrested.
"Update: unknown of extent of injuries, possible 8 - 10 pedestrians struck," Toronto police wrote on Twitter.
Police then said it was "too early to confirm the number of pedestrians struck or their injuries."
Officers were called to the scene -- on Yonge Street at the corner with Finch Avenue -- at 1:27 pm (1727 GMT), police said, without specifying whether or not the incident was deliberate.
A white rental van with a dented front bumper was stopped on the sidewalk of a major intersection, surrounded by police vehicles.
"Our hearts go to anyone affected," Prime Minister Justin Trudeau told the House of Commons.
"We are going to have more to learn and more to say in the coming hours," he said.
The incident occurred as Toronto was hosting a meeting of public security ministers from the G7 leading industrialized nations, and on the heels of a weekend gathering in the city for the foreign ministers from the same seven countries -- the United States, Japan, Britain, Germany, France, Italy and Canada.
Vehicle attacks have been carried out to deadly effect by extremists in a number of capitals and major cities, including London, Paris, Berlin, Stockholm, New York, Nice and Barcelona.
Toronto's police had not stated what they believe the driver's motives may have been in Monday's incident.
"He was going really fast," one witness, Alex Shaker, told CTV television.
"All I could see was just people one by one getting knocked out, knocked out, one by one," Shaker said. "There are so many people lying down on the streets."
Canada has largely been spared the violence of Islamist extremists, although it has not been entirely immune.
Last October, a man stabbed a police officer in the western city of Edmonton before slamming his van into a group of pedestrians, injuring four people.
And in Quebec in October 2014, a Canadian man ran over two soldiers in a parking lot with his car, killing one of them. The driver was shot dead by police when he attacked them with a knife.
And in March 2016, a Canadian who claimed to have radical Islamist sympathies attacked two soldiers at a military recruitment center in Toronto.