Toronto, Jun 27 (DPA): There was sporadic violence in the streets of Canada's largest city Saturday as at least 10,000 otherwise peaceful demonstrators fumed at the world's 20 leading powers gathered for a summit on the global economy.

Dozens of windows were smashed in the city centre and at least three police cars were set on fire. Police arrested more than 100 people Saturday and used tear gas to disperse some of the more violent protests, which they blamed on a small "mob" of anarchists.

Rocks, bottles and bricks were thrown at police officers decked out in riot gear as some groups tried to breach a security zone around the conference centre where world leaders were meeting.

"This isn't our Toronto. My response is anger," the city's mayor, David Miller, told local television station CP24.

While the violence was minimal compared to many past summits of world leaders, Toronto officials were taken aback by the display. About 12,000 police officers were out on the streets, befitting a nearly $1 billion price tag for security.

"Saturday has certainly proven to be a difficult day," said Toronto Police Chief William Blair. "I think it was shocking to every citizen. We have never seen that level of wanton criminality and random violence on our streets."

Police also stormed a park where protesters had been gathering since Friday, turning their batons on some who refused to leave, according to the Toronto Globe And Mail. Blair confirmed the action, but denied reports that Queens Park had earlier been declared a "free-speech zone".

"Canada is a free speech zone. However, there are limits to free speech," Blair told reporters. The protests were mostly made up of peaceful activists, but "a large segment of the protest (used) tactics of violence and destruction".

The conference centre where leaders of the Group of 20 bloc were meeting was sealed off behind a 3-metre-high fence.

The convoy transporting Spain's Prime Minister Jose Luis Rodriguez Zapatero from the airport was blocked by protests from reaching his hotel, sources in the Spanish delegation said.

Police decided that the route was unsafe, and Zapatero ended up in a different hotel than the one he was booked to stay at.

Most restaurants and shops in Toronto's city centre had already boarded up their windows Friday in anticipation of the protests, which included anti-globalisation, environment, labour union and aid activists.

Police were given controversial special powers to immediately arrest demonstrators within 5 metres of the perimeter fence if they refuse to identify themselves or resist a body search. Critics dubbed Toronto a "police state".

Police were bracing for some of the violence to continue through the night. More arrests were taking place "as we speak", Blair told reporters in the evening.