Police didn't immediately identify the driver, and said it was not clear whether the crash was accidental or the driver had a motive.
Here, from Sky News, is the scene where the attack took place.
The driver jumped out of the van and pointed what looked like a gun at an approaching police officer.
According to local media, at least two people were killed in the incident.
Toronto Mayor John Tory encouraged residents to remember their city's worldwide reputation for tolerance and inclusiveness - hours after a man mowed down pedestrians with a rental van, killing at least nine people and injuring several more.
The incident occurred just before 1.30pm (1730 GMT) as large crowds of office workers were on lunch breaks.
The suspect is identified as Alek Minassian, a 25-year-old from the Toronto suburb of Richmond Hill. Witnesses told the Global News the van appeared to "completely out of control" driving down the major thoroughfare in downtown Toronto.
Chemical weapons inspectors visit Syria's Douma
According to the foreign Minister of France , it could also indicate an attempt to hide the use of chemical weapons . The US, UK and France launched military strikes on Syria following the suspected attack.
"Everybody was out enjoying the sunshine and all of a sudden there was this Ryder van coming south on the sidewalk ... plowing people down", he said.
Police arrested a suspect at the scene, but earlier said it was too soon to say whether the attack in Toronto - which has a population of about 2.8 million people - had been deliberate, and if so, what the driver's motivation might have been.
In October 2017 a man in NY drove a van down a bicycle path, killing eight people. He was reportedly driving 60-70 miles per hour on Yonge street near Finch Avenue, a busy street in the north part of Toronto on April. 23.
Diego Dematos was driving on Yonge street in the opposite direction to the van and witnessed the vehicle collide with pedestrians. "The victims and their families have our deepest condolences".
The incident occurred about 30 kilometres (18 miles) away from the site where Toronto is hosting a Group of Seven foreign ministers. He hit every single person on the sidewalk. The entire stretch presented a ghastly scene as bodies lay scattered on it.
Another man said he saw one of the victims dragged along the road, leaving a trail of blood on the asphalt.
In Ottawa, Prime Minister Justin Trudeau told journalists as he entered Parliament that "our hearts go out to anyone affected" and that he was monitoring the situation closely. "Brutal" was how he described the scene, adding that there were "about five, six people being resuscitated by bystanders and by ambulance drivers".