Two women, one man and a teenager were killed when they were out for a walk when the pick-up truck hit them. A nine-year-old boy is in hospital with serious injuries.
Police said the killings were premeditated as the 20-year-old driver, identified as Nathaniel Veltman, was wearing a vest that looked like body armour.
Veltman could face terrorism charges even though he has already been charged with four counts of first-degree murder and one attempted murder.
"We believe this was an intentional act. The victims were targeted because of their Islamic faith," said the city's police chief.
In a statement, Prime Minister Justin Trudeau said on Monday: "I'm horrified by the news from London, Ontario. To the loved ones of those who were terrorised by yesterday's act of hatred, we are here for you. We are also here for the child who remains in hospital, our hearts go out to you, and you will be in our thoughts as you recover."
Sympathising with the Muslim community of Canada, he said: "We stand with you. Islamophobia has no place in any of our communities. This hate is insidious and despicable and it must stop."
London city Mayor Ed Holder called the incident "an act of mass murder, perpetrated against Muslims - against Londoners - and rooted in unspeakable hatred".
Meanwhile, Toronto Mayor John Tory announced to dim the Toronto Sign on Monday night "to mourn the London family - three generations of one family - targeted and murdered because of their Islamic faith and to stand in solidarity with our Muslim community".
He added that the city stands "with the Muslim community in London and here in Toronto in the face of this tragedy. And we absolutely reject the hatred and Islamophobia that led to this deadly violence".