Prime Minister Justin Trudeau described it as a 'tragic and senseless attack'
Police in Canada say the death toll has risen to 10 after a van was driven into pedestrians in central Toronto.
15 others have been hospitalised following the incident, which authorities say appears to have been deliberate.
25-year-old suspect Alek Minassian, from the suburb of Richmond Hill, has been arrested.
Toronto Police Chief Mark Saunders said: “We’re still looking for more witnesses.
“We’re working collaboratively not just on a local level but on a federal and provincial level. We’re looking strongly at the exact motive.”
The incident happened at around 1.25pm on Monday afternoon local time, when the van mounted the curb on Yonge Street in the city centre and struck pedestrians.
The van was stopped and the suspect arrested around 25 minutes later.
Witnesses sought to Yonge-Finch investigation~Read Toronto Police Service website update on Yonge Finch https://t.co/q5Qh47LV7R Contact police on dedicated line: 416-808-8750 or anonymous Crime Stoppers 1-800-222-8477 ^sm pic.twitter.com/dL2GBOjE3y— Toronto Police (@TorontoPolice) April 24, 2018
An Irish man from Co Clare who works on Yonge Street said: “We heard a lot of sirens and commotion coming from outside our office building just after we had come back from lunch.
“You could see shoes, blood and lots of broken glass in the road. It was horrific."
He added: “It’s a really busy part of downtown and because it’s such a beautiful day I’m sure there were even more people out and about on the streets than usual.”
Additional police have been deployed across Toronto in the wake of the incident, while officials say victims are still being identified.
In a statement, Canadian Prime Minister Justin Trudeau said: “It was with great sadness that I heard about the tragic and senseless attack that took place in Toronto this afternoon. On behalf of all Canadians, I offer my heartfelt condolences to the loved ones of those who were killed, and my thoughts for a fast and full recovery to those injured.
“I thank the first responders at the scene who managed this extremely difficult situation with courage and professionalism. They faced danger without hesitation, and their efforts no doubt saved lives and prevented further injuries."
He added the government was 'monitoring the situation closely'.
Canada's public safety minister Ralph Goodale said it appeared there was 'no national security connection' with the Toronto incident.
After speaking with top security officials - based on information to date - there appears to be NO National Security connection to the horrible event in Toronto today.— Ralph Goodale (@RalphGoodale) April 24, 2018
Tánaiste Simon Coveney said he "unreservedly condemns what appears to have been a deliberate attempt to kill and injure as many people as possible", and extended condolences to the families and friends of the victims.
He said: "While it is not yet clear what the motive was and no national security connection has yet been established, it is nonetheless a tragic and appalling incident.
"I know that the people of Canada, and of Toronto in particular, will be filled with shock and with sadness today. I want them to know that Ireland stands with the people of Toronto and with the people of Canada."
He added: "There have been no reports of any Irish citizens being caught up in the incident. If any Irish citizens do need assistance or if friends or family have concerns about a loved one, they should contact 01 408-2000."