Nearly 600 people were arrested in violent weekend protests against a G20 summit as Canadian police used tear gas and rubber bullets to break up the mobs, authorities said Sunday. Sporadic clashes between pockets of protestors and riot police flared throughout the day with 224 people detained on Sunday alone. The violence flared near a security barrier erected around the conference center where leaders of the world's top economies were gathering, and left storefronts and windows smashed and several police cars burnt out. During a raid at the University of Toronto campus, police rounded up 70 people believed not to be students. Officers recovered 'street-style weaponry' such as bricks at the scene. 'It's not the kind stuff you'd need for a shopping weekend in Toronto,' quipped an officer. Police also moved on a seemingly peaceful sit-in outside a film studio turned into a makeshift detention center to arrest a 'known anarchist' hiding in the group, according to the Integrated Security Unit. At daybreak, Canada's largest city was on tenterhooks. Police in riot gear kept vigil at intersections, while city crews swept up broken glass and wiped graffiti off buildings and streetcars. Residents were back walking their dogs, joggers trotted through parks, but most downtown shops and restaurants remained closed. 'Slowly the city is returning to a level of normalcy,' said Sergeant Tim Burrows, spokesman for the Integrated Security Unit responsible for securing the meeting of the world's 20 leading economies. Overnight, the subway was shut down, and area hotels and hospitals were locked down as police clashed with militants. Tear gas was used against a black-clad crowd of demonstrators that refused to disperse following a warning after they pelted many officers with bottles and stones, the police chief said. Manhole covers were also welded shut for fear protestors were hiding in sewers. Police arrested four men after they popped out from one manhole behind police lines. But Toronto police used Twitter to deny rumors that rioters had breached the security barrier erected around the conference center where G20 leaders were gathering. 'We have never seen this level of wanton criminality on our streets,' Toronto police chief Bill Blair told reporters, after four police cars were set on fire and windows in the financial district were smashed. Some 30,000 people, according to rally organizers, marched against the G20 summit on Saturday demonstrating in favour of social causes, in what was a largely peaceful rally until violence erupted on its fringes. The main body of the march was a well-marshaled event, led by older activists and organized labor, but splinter groups of young hardliners scuffled with riot officers and set patrol cars ablaze. The violence 'was shocking to every citizen,' Blair said, warning of more arrests to come of people already 'known to police.' Government spokesman Dimitri Soudas said: 'Free speech is the principle of our democracy.' But he denounced what he described as 'a bunch of thugs that pretend to have a difference of opinion with policies and instead choose violence to express those so-called differences.' 'People do disagree on issues. Leaders that meet all the time don't necessarily see eye-to-eye but you don't see people burning up police cars and breaking windows,' he said. David Martin, Greenpeace's climate change campaign coordinator, said the group behind the violence has 'no base and no credibility.' 'Yet this very small group of irresponsible people... has taken media attention away from the coverage of essential issues, like climate change, foreign aid... maternal health and so on,' he said. 'I am not a pacifist. I am for civil disobedience, but not for this kind of crazy violence. 'The Black Bloc people say they want social change, but in fact they have an adrenaline addiction to violence,' he added. 'They live for these international events but tomorrow they'll disappear and leave us to do the hard work, which they've now made harder for us.' Canada spent more than a billion dollars to secure this week's back-to-back G8 and G20 summits in the Toronto area, hoping to avoid the serious street battles that marred recent gatherings of these global forums. Thousands of police reinforcements backed by riot officers on horseback and spotter helicopters were drafted into the city center, much of which was sealed off behind concrete and steel barriers. 