Protesters set a police car on fire at Bay and King streets(Mark Blinch-Reuters)

Protesters set a police car on fire at Bay and King streets

At least 150 people were arrested as violence broke out after thousands of anti-G20 protesters marched through downtown Toronto on Saturday, prompting police to use tear gas in the city for the first time.

“I am profoundly disappointed in the criminal acts which have taken place,” Toronto police Chief Bill Blair told reporters.

"We have seen windows broken and police cars burned. It is very regrettable that such vandalism and violence could not be prevented.

"I want to assure you that the persons responsible will be held accountable.”

Four police vehicles were set ablaze, store and bank windows were smashed and much of the area was put under security lockdowns. At least 150 people were arrested, the Integrated Security Unit said

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Throughout the evening, police moved people east along Queen Street in the downtown core.

Shortly before 8 p.m., a police vehicle that had been damaged earlier in the day on Queen Street, just east of Spadina Avenue, was torched. Police in riot gear descended near the burning wreckage to push people back.

Two police cruisers were torched earlier at the corner of King and Bay streets in the heart of the financial district, sending plumes of black smoke into the air. As one vehicle burned, protesters surrounded officers who were trying to protect the second car, CBC reporter Amber Hildebrandt reported on Twitter.

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Blair blamed the destruction on violent "anarchists" and said several of their leaders were arrested.

Blair said that throughout the day some of his officers were pelted with rocks, bricks and bottles, spat upon and assaulted, but none suffered major injuries.

Blair confirmed that tear gas was deployed once — for the first time in the city's history — "after a warning was given to the public about its impending use." But he denied that rubber bullets were used.

The size of the protest crowd was estimated as high as 10,000.

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As the evening wore on, the area around the Ontario legislature at Queen's Park emerged as a major focal point. Several hundred police in riot gear — many on horseback — ringed government buildings and lined streets in the area, as well as the nearby park grounds.

Police repeatedly moved toward groups of demonstrators to move them back. At one point, many protesters were arrested.

Blair said police were sent to that area because many members of a mob were seen going there to change clothes.

Asked whether police were slow to respond to the violence, Blair said a mob had emerged from the initially peaceful protest and broke into several groups.

"It did take us some time to move our resources," Blair said.

Blair later said police are reviewing their tactics, "what worked and what didn't work as well."

A spokesman for Prime Minister Stephen Harper said free speech is a principle of our democracy "but the thugs that prompted violence earlier today represent in no way, shape or form the Canadian way of life."

Toronto Mayor David Miller also blamed a small group of "thugs" for the violence.

“People are calling them protesters. That is not fair to the people who came to protest,” he said.

Police said there have been only minor injuries.

With the violence escalating in the heart of Canada's largest city, the entire area around the G20 summit site at the Metro Toronto Convention Centre — enclosed by concrete barricades and fences — was under a security lockdown.