Toronto, Canada (CNN) -- Police said they were preparing for more disruptions Sunday after groups of protesters burned cars, hurled bricks and smashed windows as they tried to penetrate the fence surrounding the G-20 summit.
"I do not believe that the individuals bent on vandalism and violence in our city have finished with their intent, so we will remain vigilant," Toronto Police Chief Bill Blair said Saturday night.
More than 300 people were arrested Saturday, and 412 have been arrested since June 18, said Jillian Van Acker, spokeswoman for the Integrated Security Unit. There were no reports of serious injuries to either protesters or police, she said.
Blair told reporters that packs of disruptive demonstrators infiltrated peaceful protests in order to cause chaos and distract police.
"These criminals rely on the anonymity of hiding in a larger group of the curious and the naive," he said.
At least four police cars went up in flames and smoke during hours of confrontation.
Protesters left behind broken windows and graffiti. Blair said police used tear gas after warning a group of protesters "engaged in acts of destruction" Saturday.
But not all encounters between police and protesters were hostile.
At one intersection the crowd danced and chanted, "You're sexy, you're cute, take off your riot suit!"
More aggressive groups of demonstrators moved from intersection to intersection, trying to circumvent police and get to the security fence protecting the summit meeting.
"The fence is a symbol that they can build a fence and spend a billion dollars on their agenda. The fence is a symbol of what's wrong with this country," protester Rolf Gerstenberger said.
Everywhere the protesters went, police were waiting to head them off, in some cases with individual blasts of pepper spray, tear gas and bean bag pellets, according to the summit's security unit.
Authorities said the fence was not breached Saturday.
Behind it, world leaders began an economic summit focusing on recovery from the global financial crisis.
As they prepared for Sunday's scheduled meetings, the U.N. chief urged them to remember that the world's poor need help making ends meet now more than ever.
"Let me emphasize this evening that, under any circumstances we must not balance budgets on the backs of the world's poorest people," U.N. Secretary-General Ban Ki-moon said.
Ban said countries should invest in agriculture, green recovery jobs and health.
On Saturday, European Union Commission President Jose Manuel Barroso stressed that European leaders were already taking action to strengthen their economies and deal with a weakening euro caused by public debt woes.
"There should be no doubts. Europe will do whatever it takes to assure the financial stability of the euro," he told CNN.
This weekend's meetings come on the heels of the two-day G8 summit outside Toronto, where the leaders of the United States, Canada, France, Germany, Britain, Italy, Japan and Russia also focused on recovery from the global economic crisis. The group also made note of other thorny issues in their final statement issued Saturday, singling out Iran and North Korea for criticism.
CNN's Jeanne Meserve and Ali Velshi contributed to this report.