With sirens still blaring at mid-night Saturday, downtown Toronto was in a state of high alert, after an anti-G20 protest turned into a riot, with at least 150 people arrested by police.

The demonstration turned violent after more than 10,000 protesters marched through downtown Toronto on Saturday, and police used tear gas in the city for the first time.

"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," spokesman Dimitri Soudas of the Prime Minister's office said in a statement.

The spokesman was not the only official to use the word "thugs" to describe those violent protesters.

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

At least four police vehicles were set ablaze, and at least 150 people were arrested.

Collin Chu, an engineer who watched the protesters march on streets near Queen West and Spadina streets in the center of the city, said the protesters seemed to target a number of U.S.-based chain shops.

"I saw a Starbucks shop was smashed at the corner of the Queen West," he told Xinhua, adding that for many years, he has never found Toronto could be in such a "scary" condition.

"We want to showcase our financial achievements, but safety should be the top priority," said Chu. "It is very embarrassing."

"I can only call them criminals and they should be arrested," David Miller, the mayor of Toronto, told Xinhua on Saturday.

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

More than 10,000 people took to the streets of Toronto as part of a massive anti-G20 protest. The protesters smashed shop windows in the city's downtown core.

The mayor told media on Thursday that he was confident the Toronto G20 summit would be a "safe" one. Canada has spent more than 1.2 billion Canadian dollars (1.15 billion U. S. dollars) on security for the summit.

However, what what happened on Saturday seems to have left some doubt over the security of the summit.

The G20 security zone was put under lockdown, so were several other locations including the Eaton Center shopping mall, Sick Kids Hospital and Toronto General Hospital.

The shopping mall of Eaton Center is only two blocks away from the intersection of Bay and Adelaide, where the protest turned into a riot.

There were at least three Starbucks smashed and every single window of a major bank was poked a hole.