Up to 10 pedestrians have been struck by a van that witnesses said mounted the kerb in Toronto, say police.
The vehicle fled the scene after the incident at a busy intersection in the north of the Canadian city, police say.
The van was located several blocks away, and the driver was taken into custody, the BBC has confirmed.
Police have yet to confirm any fatalities, but witnesses told Canadian media a number of people appeared to have been killed in the incident.
Authorities have no cause or reason for the collision, the BBC’s Jessica Murphy confirmed at the scene.
She said there are what appear to be at least two bodies covered by orange tarp, but reports of deaths are still unconfirmed.
“A lot of people didn’t see the attack but were shocked something like this would happen on a busy but generally calm street,” she added.
Toronto’s Sunnybrook hospital says it has received seven patients so far, and has locked down its emergency centre as an “added precaution”.
The incident happened at Yonge Street and Finch Avenue at 13:30 local time (17:30 GMT) on Monday.
The crime scene encompassed a 2km (1.24 mile) stretch of Yonge Street in Canada’s largest city.
It happened about 18 miles (30km) from the downtown area where the Group of Seven foreign ministers of industrialised nations – including Canada, US, Britain, France, Germany, Italy and Japan – were holding meetings.
Pictures apparently taken at the scene show police armed with rifles and paramedics treating injured people.
One orange bag, which appears to contain a body, is seen being loaded on to an ambulance.
“Reports were that a white van mounted the kerb, drove down the sidewalk at southbound Yonge, south of Finch, and struck eight to 10 people possibly, the numbers aren’t confirmed yet,” Toronto police spokesperson Gary Long told Canadian broadcaster Global News.
Toronto police spokeswoman Jenifferjit Sidhu told CBC News that the vehicle was stopped by police.
No description was given of the driver, including name, age or gender.
Canadian Prime Minister Justin Trudeau tweeted from Ottawa: “Our thoughts are with all those affected by the terrible incident at Yonge and Finch in Toronto.”
“Thank you to the first responders working at the scene – we’re monitoring the situation closely.”
Speaking in broken English, one witness told local media: “I saw there’s a white truck just go to the sidewalk and so many people just are shouting, ‘stop the car!’
“But he just keep moving and he hit some people and three people lie down there and they didn’t move at all.”
One witness told City News that the driver, whom he identified as a man, was driving on the kerb “hitting anything that comes in the way”.
“People, fire hydrants, there’s mail boxes being run over,” said the unnamed man, who said he was driving behind the van during the incident.
As the van continued, the man said he honked his horn to try to warn pedestrians.
“I witnessed at least six, seven people being hit and flying in the air, like killed, on the street,” he said.
It looked like something from “a movie”, he added.
Toronto Mayor John Tory called it “a very tragic incident”.
He added he has “offered any and all assistance that the city can provide to the police to help this investigation”.
The United States and Europe have seen an increase in driving attacks in recent years.
In October 2017 a man in New York drove a van down a bicycle path, killing eight people.