500 held as anti-G20 protesters overwhelm the streets of Toronto

• Police arrest a protestor at a rally outside the temporary G20 police detention center

The latest clashes occurred as several hundred protesters marched on a temporary detention centre for demonstrators held in riots on Saturday during which police used tear gas against the public for the first time ever in Canada's most populous city.

A police spokeswoman confirmed that officers yesterday fired what are known as muzzle blasts, or "individual applications of tear gas" that are used typically against individuals at close range.

A witness in front of a former film studio in Toronto's port area where police are detaining protesters said he heard a loud "thunk" and saw clouds of smoke billow before police charged, scattering the crowd.

The weekend violence started on Saturday afternoon after groups of masked anarchists broke away from a larger, peaceful demonstration against the Group of 20 summit of rich and emerging economies.

Protesters, many dressed in black gear, smashed windows of downtown stores and banks and torched police cars in a protest that police finally brought under control with tear gas and mass arrests.

"What we're prepared for today is more of what we saw yesterday," a police spokesman said.

"We would like to see demonstrations remain peaceful."

After a day when police admitted losing control of a violent and fast-moving crowd on Saturday, the arrests came fast yesterday.

Among those detained, for charges ranging from mischief to assaulting police, were four people who climbed through the sewer system and emerged near the lock-down area where world leaders were attending the summit.

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, and police said they seized weapons, including bricks, rocks and sticks.

"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, said.

Anti-G20 groups started demonstrating in Toronto before the summit, which followed a smaller meeting of Group of Eight industrial nations in a resort town north of Toronto.

The security bill is set to come in at about $1 billion.

Such international meetings have been the target of protest groups for years, including demonstrations that disrupted trade talks in Seattle in 1999.