TORONTO -- Police raided a university building and rounded up more protesters Sunday in an effort to quell further violence at the global economic summit after black-clad youths rampaged through the city, smashing windows and torching police cruisers.
Police said they have arrested more than 560 demonstrators, many of whom were hauled away in plastic handcuffs and taken to a temporary holding center constructed for the summit.
Despite the violence, no serious injuries were reported among police, protesters and bystanders, Toronto Police Constable Tony Vella said Sunday.
Thousands of police in riot gear formed cordons to prevent radical anti-globalization demonstrations from breaching the steel and concrete security fence surrounding the Group of 20 summit site.
Security was being provided by an estimated 19,000 law enforcement officers drawn from across Canada, and security costs were estimated at more than $900 million.
Prime Minister Stephen Harper deplored the actions of a "few thugs" and suggested the violence justifies the cost. Harper has been criticized for the security price tag.
"I think it goes a long way to explaining why we have the kind of security costs around these summits that we do," Harper said.
Toronto Police Sgt. Tim Burrows said police made at least 70 arrests in a Sunday morning raid on a building on the campus of the University of Toronto, where they seized a cache of "street-type weaponry" such as bricks, sticks and rocks.
"We think we put a dent in their numbers with this and with the arrests that happened overnight," Burrows said.
The disorder and vandalism occurred just blocks from where U.S. President Barack Obama and other world leaders were meeting and staying.
"What we saw yesterday is a bunch of thugs that pretend to have a difference of opinion with policies and instead choose violence to express those so-called differences of opinion," Harper's chief spokesman Dimitri Soudas said Sunday.
The streets of downtown Toronto were quiet at daylight, but protesters gathered Sunday morning at a park near the detention center -- about 2 1/2 miles east of where the leaders are meeting.
Police adopted a more aggressive strategy Sunday by going into the crowd to make arrests, compared to the previous day when they stood back as protesters torched four police cars and broke store windows.
Previous global summit protests have turned violent. In 1999, 50,000 protesters shut down World Trade Organization sessions in Seattle as police fired tear gas and rubber bullets. There were some 600 arrests and $3 million in property damage. One man died after clashes with police at a G-20 meeting held in London in April 2009.
At the September G-20 summit in Pittsburgh, police fired canisters of pepper spray and smoke and rubber bullets at marchers.