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 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," 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 1/2 miles (four kilometers) 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 at the protest near the detention center 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 that chased protesters down the street. Vella said it was not tear gas.
About 100 demonstrators chanted, "The whole world is watching! The whole world is watching!'"
Edward Canavan said he was walking along as protesters were rioting and burning cars on Saturday. He said he happened to see a box of oranges, which he didn't realize belonged to officers, and grabbed one. He was arrested and detained at the temporary jail from 2 a.m. until about noon.
"I was just in the wrong place at the wrong time," said Canavan, as he left the detention center, protesters cheering in the background.
Canavan said he was treated well and offered a sandwich by officers, but he added there wasn't much room to walk around.
"There was a place to stand in a cage, about six or eight people in a cage," said Canavan as he described the conditions.
Bridie Wyrock, 20, from Cleveland, Ohio, said she was arrested for public mischief for sitting on a street in the financial district. Wyrock, held for 19 hours before being released, said there wasn't enough toilets and said people were resisting detention, but said police treated most people with respect.
"They put us in cages, blocked off on all three sides," Wyrock said. "It was cold and dirty."
Burrows said many of those involved in the violent protests were Canadian. He added that authorities had known of their plans for some time.
Thousands of police headed to Toronto to reinforce security there after the smaller Group of Eight summit ended Saturday in Huntsville, Ontario, about 140 miles (225 kilometers) away. Security was being provided by an estimated 19,000 law enforcement officers drawn from across Canada, and security costs were estimated at more than US$900 million.
Saturday's protests began with a peaceful march, sponsored by labor unions, that was the largest demonstration planned during the summit weekend. Its organizers had hoped to draw a crowd of 10,000, but only about half that number turned out on a rainy day.
The black-clad demonstrators broke off from the larger crowd of peaceful protesters and began torching police cars and smashing shop windows.
Toronto Police Chief Bill Blair said the goal of the militant protesters was to draw police away from the security perimeter of the summit so that fellow protesters could attempt to disrupt the meeting.
Some police officers were struck by rocks and bottles and assaulted, but none was injured badly enough to stop working, Blair said.
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.