The arrest tally from rioting in downtown Toronto climbed above 500 on Sunday, including four 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 Sunday 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 10-foot 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 on Sunday, and police said they seized weapons, including bricks, rocks and sticks.

The riots started on Saturday afternoon after groups of masked anarchists broke away from a larger, peaceful demonstration that was protesting against the G20, which ends its meeting on Sunday.

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.

On Sunday, protesters carrying banners saying Free Our Comrades marched on the temporary detention center 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, Dimitri Soudas, 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.

Meetings like these 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.

(Additional reporting by David Ljunggren and Claire Sibonney; Editing by Janet Guttsman)