The violence prompted police to place Canada's largest city in a virtual state of lockdown -- with subway service to the downtown core halted for much of the day, major businesses and hospitals shuttered, and officers with shields holding back agitated crowds near the security zone where world leaders of the G20 were meeting.
Anarchist protesters are being blamed for much of Saturday's violence, which left several businesses and banks vandalized and at least four police cruisers wrecked.
Hundreds of arrests have been made in connection with the protests, with various reports estimating 400 - 600 in total so far.
Dressed in black and with their faces concealed by balaclavas, the violent mob broke away from the mostly peaceful throng of 10,000 protesters before smashing bank windows and targeting U.S.-based businesses.
A Starbucks coffee shop had its front window smashed, and three Canadian banks were vandalized, with damning messages spray-painted on the walls. Several business storefronts along Yonge Street -- one of Toronto's busiest streets -- were vandalized, with reports of windows smashed, merchandise looted, and in some cases, feces thrown into the window displays.
Toronto Mayor David Miller blamed a "small group of criminals" for the destruction and violence.
"I just want to say to Torontonians directly: Please stay calm, don't be disturbed," Miller said at a news conference.
Police -- whether they were on bikes, in riot gear, or on horse-back -- responded in force as things took a turn for the worse.
In the heat of the protests, officers threatened to launch tear gas into the crowds. One incident saw a police officer standing atop a perch, pointing a tear gas grenade gun at the crowd before he was talked down by the throng who chanted, "Put the gun down."
Toronto police chief Bill Blair later confirmed in a news conference that tear gas had been deployed once during the protests -- the first time ever in the city -- after police had given a warning to the public. But although various reports stated that officers had fired rubber projectiles in an effort to hold back aggressive protesters, Blair denied the use of rubber bullets.
Elsewhere, two other abandoned police cruisers had their windows smashed and bodies spray-painted as a group of gawking spectators watched and filmed the spectacle. A protester sprayed the words "state" under the police logo emblazoned on the side of one car before spray-painting himself in red.
"I can understand why someone would do that considering the way police have antagonized people today," said Justin Geever, better known as Justin Sane of the political, punk-rock band Anti-Flag. Sane was out among protesters in Toronto.
"The police are not on our side," he said. "They're goons that are protecting multinational corporate heads.
"I think it's important for the corporate rulers of this world to understand that people aren't just going to take what they spoon-feed us," the singer added.
Other protesters weren't as impressed, with some denouncing the violence as counter-productive.
"I want us to focus on what we're here for, for all the groups that are here protesting," said a demonstrator who didn't want to be identified.
It was an ugly turn of events for a day that began relatively peacefully. Labour groups, maternal health advocates, and anti-war activists took the streets around 1 p.m. in an organized manner before splinter groups broke away from the mass and instigated the violence.
In a statement, federal Public Safety Minister Vic Toews said the images of violence and upheaval were "truly shocking to Canadians."
Toronto Emergency Medical Services reported treating three people for injuries during the protests. At least two protesters were seen bleeding from the head after a run-in with a wall of police at Queen's Park, near Ontario's provincial legislative building.
Chief Blair said his officers had also suffered minor injuries from attacks by protesters, but none had been called off duty.
With files from CBC News