Ten people have been killed and 15 injured after a man drove a van into pedestrians in Toronto, police said.
The suspect has been named by police as Alek Minassian, 25.
A man was arrested several streets away following a tense standoff with officers on the street.
Bystander videos appeared to show the driver pointing an object at the officers, who could 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". He said separate hotlines had been set up for victims' families and for witnesses.
City police chief Mark Saunders told a news conference that the incident appeared to be deliberate but the motive was still unknown. He said Alek Minassian, from the Toronto suburb of Richmond Hill, was not previously known to them.
Canadian public safety minister Ralph Goodale, in a tweet thanking the emergency services, referred to the incident as a "horrific attack".
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.
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."