TORONTO - Police raided a university building and rounded up more protesters yesterday 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 cars.

Police said they had arrested more than 600 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 yesterday.

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.

Toronto Police Sgt. Tim Burrows said police made at least 70 arrests in a 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 President 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," Prime Minister Stephen Harper's chief spokesman Dimitri Soudas said yesterday.

The streets of downtown Toronto were quiet at daylight, but protesters gathered yesterday morning at a park near the detention center - about 2.5 miles east of where the leaders are meeting.

Police adopted a more aggressive strategy by going into the crowd to make arrests than the previous day when they stood back as protesters torched four police cars and broke store windows.

Plainclothes police jumped out of an unmarked van, grabbed a protester off the street and whisked him away in the vehicle. The protest was then quickly broken up by riot police, who set off a warning device that created a cloud of smoke chasing protesters down the street.

Burrows said many of those involved in the violent protests were Canadian. He added that authorities had known of their plans for some time - long enough, for example, to move the Blue Jays-Phillies series to Philadelphia.

"We're not sure we have the leaders, but we have a large proportion of those people and the people who decided they wanted to be influenced by these violent protesters and join with their cause," Burrows said. "A lot of them were home grown. There's a lot of Canadian talent in the group."