Nine people have been killed and 16 injured after a man drove a van into pedestrians in Toronto, police said.
The driver fled the scene of the incident, at a busy junction in the north of the Canadian city, but was arrested several streets away.
The arrest followed a tense standoff with officers on the street.
Bystander videos appeared to show the driver pointing an object at the officers, who can be heard shouting at him to get down.
The man was then detained without any shots being fired.
Toronto deputy police chief Peter Yuen asked for witnesses to come forward and said there would be "a long investigation".
Police would not say if they believed the act was deliberate but Canadian public safety minister Ralph Goodale, in a tweet thanking the emergency services, referred to it as a "horrific attack".
Reza Hashemi, who owns a video shop on Yonge Street, told the BBC he heard screaming on the other side of the road.
He said the white rental van had repeatedly mounted the pavement and run into people.
Van rental company Ryder System Inc confirmed that one of its vehicles was involved and said it was co-operating with authorities.
The incident occurred 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.
About 18 miles (30km) away in the city centre, foreign ministers of the G7 leading industrialised nations - Canada, the US, Britain, France, Germany, Italy and Japan - were holding meetings.
Pictures apparently taken at the scene showed armed police and paramedics treating the injured. One orange bag, which appeared to contain a body, was loaded on to an ambulance.
Toronto police spokeswoman Jenifferjit Sidhu told CBC News that the vehicle was stopped by police. No description was given of the driver.
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."
One witness told City News that the driver was "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 sounded 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.
Toronto Mayor John Tory called it "a very tragic incident". He said he had "offered any and all assistance that the city can provide to the police to help this investigation".
The US 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.