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More than 400 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 said at a news conference.

"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. The Integrated Security Unit said 412 people were arrested.

Throughout the evening, police moved people east along Queen Street. Police were still trying to move small groups out of the downtown core early Sunday, hours after most demonstrators had gone home.

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.

Officers assaulted, chief says

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 Canada's largest city — "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.

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.

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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 of vandals.

"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 political summit site at the Metro Toronto Convention Centre — enclosed by concrete barricades and fences — was under a security lockdown. 

Transit halted

The boundaries include Wellington Street to the north, Lakeshore and Bremner boulevards to the south, Windsor Street and Blue Jays Way to the west and a section of Bay Street to the east.

Downtown subway service remained suspended, with no service in the loop between Bloor and St. George stations, and there were no streetcars or bus service in the area.

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A police car burns after G20 summit protesters set fire to it in downtown Toronto on Saturday. ((Canadian Press) )

GO train service into and out of Union Station has also been stopped.

The airspace over downtown Toronto was closed, as was the Gardiner Expressway, one of the city's major traffic arteries.

Some hospitals, hotels, businesses and the tourist attraction the Eaton Centre were also under lockdown.

When the demonstration started just after 1 p.m. ET at the legislature, the mood was upbeat and peaceful. Protesters danced, clapped and chanted while carrying signs, flags and umbrellas as they first moved south down one of the city's main thoroughfares, University Avenue.

But the mood turned ugly as members of splinter groups broke away in attempts to move toward the summit site near Lake Ontario. They used a tactic known as the Black Bloc, in which protesters wearing black hoodies, masks, balaclavas and skateboard helmets engage in violent behaviour.

At one point, protesters smashed windows on both sides of Yonge Street in the normally busy shopping area in the Yonge-Dundas corridor. Witnesses said rioters smashed the information booth of the large Future Shop electronics store.

"It's a sizable crowd and getting bigger and bigger" in some areas, the CBC's Steven D'Souza reported. "It doesn't look too dangerous, but it is gathering momentum where I am right now."

'Why do they come here and make problems and make problems for every store?'— Myung Hwon Yang, store owner

Steven Connor, who works at the Hard Rock Café near the Eaton Centre, said several glass storefronts were smashed, with some vandals using street pylons and road equipment.

No one seemed to be injured, Connor said.

"There's been a lot of damage in the downtown core," the CBC's Michael Serapio reported.

Michael Hyatt, who was at a gym near Yonge and Dundas streets, said the protesters seemed to target a number of U.S.-based chains.

"It is pretty horrible what they have done to a lot of the stores here. They’ve destroyed the windows at an American Apparel — they destroyed all the windows and pulled out the mannequins and [threw] feces into the store.

"It stinks and it is unbelievable. Foot Locker is destroyed. Pizza Pizza is destroyed. They’ve kind of gone up the street and picked at every U.S. vendor they could find. It’s really kind of sad."

Bricks, rocks thrown

On many downtown streets, many store windows were either broken with bricks or rocks, or defaced with graffiti. One bank on Queen Street West had its windows smashed and a CBC van was damaged.

One convenience store owner said he was determined to stay open.

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Protesters, including some wearing masks, take part in Saturday's march. ((Nazim Walji/CBC))

"It's important for countries to meet, but an island area is better. Why do they come here and make problems and make problems for every store?" Myung Hwon Yang told CBC News.

In one incident, a man dressed in red was arrested and then dragged screaming into a police van. A large crowd gathered, denouncing police and chanting, "Let him go."

Miller said police had made thorough preparations ahead of the summit and did a "commendable job under difficult circumstances."

"I'm sure there have been small moments where perhaps there’s some tensions between a crowd and the police," Miller told reporters. "In the broad brush, I think we should be very confident in their work."

He added that Blair had been very clear in recent days that authorities would facilitate a lawful, democratic protest, while expressing concern about groups who "come here just to perpetrate violence."

"We've seen that his concern was justified," Miller said.