The arrest tally from rioting in downtown Toronto climbed above 500 today, including four people who climbed through the sewer system and emerged near the lock-down area where world leaders were attending the G20 summit.
Police said they hoped protests planned for today would be quiet, but after a day when they admitted they had lost control of a violent and fast-moving crowd, the arrests came fast.
"What we're prepared for today is more of what we saw yesterday," a police spokesman said. "We'd like to see demonstrations remain peaceful."
Four people were detained in the middle of the night as they emerged from a manhole near the three-metre fence sealing off the area where the Group of 20 rich and developing nations are meeting. Police said they were urgently sealing sewer access near the zone.
A "large number" of people were detained in a raid at the University of Toronto's downtown campus today, and police said they seized weapons, including bricks, rocks and sticks.
The riots started yesterday afternoon after groups of masked anarchists broke away from a larger, peaceful demonstration that was protesting against the G20, which ends its meeting today.
They smashed the windows of stores and banks and torched police cars in a protest that police finally brought under control with tear gas and mass arrests. Banks, coffee shops and small stores were also targets and protesters looted at least one retailer, storming out with clothes and fashion mannequins.
Toronto police chief Bill Blair admitted last night police had struggled to control the crowds.
"We have never seen that level of wanton criminality and vandalism and destruction on our streets," he told a news conference. "There are limits to free speech, and these limits really end when it infringes on the rights and the safety of others."
Today, protesters carrying banners saying "Free Our Comrades" marched on the temporary detention centre set up near Toronto's port area. Police fired bean bags and tear gas to disperse them.
Those arrested face charges ranging from mischief to assaulting police, police said.
"What we saw yesterday ... is a bunch of thugs that pretend to have a difference of opinion with policies and instead choose violence in order to express those so-called differences of opinion," a spokesman for Canadian prime minister Stephen Harper, told a news conference.
Anti-G20 groups have been demonstrating in Toronto leading up to the summit of rich and emerging economies, which follows a smaller meeting of Group of Eight industrial nations. The security bill is set to come in at around $1 billion.
Similar summits have been the target of protest groups for years, including demonstrations that disrupted trade talks in Seattle in 1999. The Toronto meetings went ahead as scheduled.