Black-clad demonstrators broke off from a crowd of peaceful protesters at the World Summit, torching a police cruiser in the financial district and smashing windows with baseball bats and hammers.
Police with shields and clubs earlier pushed back another small group of demonstrators who tried to head south toward the security fence surrounding the perimeter of the Group of Twenty global economic summit site. Some demonstrators hurled bottles at police.
"This isn't our Toronto and my response is anger," Toronto Mayor David Miller told CP24 television. "Every Torontonian should be outraged by this."
Previous major world summits have attracted massive, raucous and sometimes destructive protests by anti-globalization forces. But so far the Canadian demonstrations have been smaller, with rain apparently discouraging some protesters.
Organizers of Saturday's demonstration were hoping to draw a crowd of 10,000, but only about half that number turned out.
The roving band of protesters in black balaclavas broke shop windows for blocks with baseball bats and hammers. They also broke windows at police headquarters.
Police spokeswoman Jillian Van Acker said officers employed tear gas a short distance from there.
Police in riot gear and riding bikes formed a blockade, keeping protesters from the security fence a few blocks south of the march route. Police closed a stretch of Toronto's subway system along the protest route and the largest shopping mall downtown closed after the protest took a turn for the worse.
Tons of police cars headed to Toronto to reinforce security there after the smaller Group of Eight summit ended in Huntsville, Ontario. The vandalism in Toronto's downtown core occurred just blocks from where President Barack Obama and other world leaders are meeting and staying.
Toronto Police with shields and clubs earlier pushed back a small group of protesters who tried to head south toward the security fence around the site of the G-20 summit. Some demonstrators hurled bottles at police.
As of Saturday afternoon, 40 summit-related arrests had been since June 18, police said, with security being provided by an estimated 19,000 law enforcement officers drawn from all regions of Canada. The security costs are estimated at more than US$900 million.
Toronto Police Sgt. Tim Burrows said before Saturday's protest that authorities were quite pleased that the protests have been tame so far compared to what they've seen at previous summits. Hundreds of protesters moved through Toronto's streets Friday, but police in riot gear blocked them from getting near the summit security zone downtown.
Burrows said police will be watching splinter groups within the protest who he said have promised violence.
"From what we've seen in previous summits we're quite happy with what our protesters are bringing to the table with the exception of a few," Burrows said before the vandalism.
"They've protested peacefully. There have been a few people that have decided that they wanted to step out of line and test our response and they found themselves where they should be."
Ontario's provincial government quietly passed a regulation earlier this month allowing police to arrest anyone who refuses to show identification or submit to searches if they come within five meters (five yards) of the security fence.
Toronto's downtown core resembles a fortress, with a big steel and concrete fence erected along several blocks to protect the summit site.
Previous protests during global meetings have turned violent. In 1999, 50,000 protesters shut down World Trade Organization sessions in Seattle as police fired tear gas and rubber bullets. There were some 600 arrests and $3 million in property damage. At a G-20 meeting, held in London in April 2009, thousands protested, and one man died after a confrontation with police.
At the last G-20 summit in September, police fired canisters of pepper spray and smoke and rubber bullets at marchers in Pittsburgh.
Associated Press Writers Ian Harrison and Charmaine Noronha contributed to this report.