Police said they have arrested more than 500 demonstrators, many of whom were hauled away in plastic handcuffs and taken to a temporary holding center constructed for the summit.
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 50 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," Prime Minister Stephen 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 2.5 miles (four kilometers) east of where the leaders are meeting.
Police went into the crowd and made some arrests, adopting a more aggressive strategy than the previous day when they stood back as protesters torched four police cars and broke store windows.
These Tea Party folks are just a shudder, ain't they Milky?