A police car burns after activists and protesters set it on fire along the streets of downtown Toronto. (Nathan Denette/Canadian Press)

Violence marked the first day of the G20 summit in downtown Toronto as a group of protesters broke off from other demonstrators, creating havoc in parts of the city. The so-called Black Bloc anarchists -- who promote violent confrontation with authorities -- tried repeatedly to break into the secure zone where leaders of the G20 are meeting.

Those unsuccessful attempts eventually led to smashed windows and spray-painted walls. Four police cars were set alight, 480 people were arrested, and hospitals and the Eaton Centre shopping mall were locked down.

The violence prompted police to ramp up their security tactics. Toronto police Chief Bill Blair confirmed that tear gas was deployed once -- for the first time in Canada's largest city -- "after a warning was given to the public about its impending use." But he denied that rubber bullets were used.

Police are warning that Sunday will likely bring more of the same and anticipate more demonstrations. "There will be more violence Sunday because they have not achieved their objectives yet," Blair told CBC News.

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