Toronto police chief Bill Blair admitted police had struggled to control the crowds, and had used tear gas on one occasion, after warning people to stay away from trouble spots.
"We have never seen that level of wanton criminality and vandalism and destruction on our streets," he told an evening news conference.
"There are limits to free speech, and these limits really end when it infringes on the rights and the safety of others."
At least 130 people were arrested, including some Blair believed were ringleaders of the rioting that started when several hundred anarchists broke away from a large, peaceful demonstration against the top-level meeting.
During an afternoon and night of rioting, police cars were set ablaze in at least two areas, including the city's Bay Street financial core, while protesters on hip Queen Street smashed storefronts and damaged media trucks.
By evening, some protesters had advanced to the metal fence sealing off the area where heads of state from the Group of 20 industrialized and developing countries were meeting.
A downtown hotel was locked down, with riot police outside. Banks, coffee shops and small stores were also targets and protesters looted at least one retailer, storming out with both clothing and the white arms and legs of fashion mannequins. "A relatively small group of people ... came clearly with the intent of damaging property and perpetrating violence," Toronto Mayor David Miller told a news conference.
"They're criminals that came to Toronto deliberately to break the law."
The chaotic protests marked a rare outbreak of violence in Toronto, a city generally known for its civility. But there have been similar protests at previous international meetings, usually from small groups who make their intention to cause trouble known well in advance.
As the protests escalated, a Reuters reporter said police charged the crowds to seize individuals and remove them from the group, while police on horseback moved around the perimeter, herding the group through the park where the protest began about six hours earlier.
The reporter said police also fired oversized plastic bullets in the effort to clear the park, although Blair said he was not aware that plastic bullets had been used.
The initially peaceful march numbered in the thousands when it began in the early afternoon, organized by labor groups that say G20 economic policies favor the rich.
Anti-G20 groups have been demonstrating in Toronto leading up to the summit of rich and emerging economies, which follows a smaller meeting of Group of Eight industrial nations in the Ontario resort town of Huntsville.
Canada has budgeted more than C$1 billion ($970 million) for security for the two summits.
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