TORONTO - Police cracked down hard on G20 dissent Sunday in the wake of violent protests that saw downtown Toronto transformed into part police state, part riot zone over the weekend.

Scenes of unopposed militants using so-called Black Bloc tactics to wreak havoc along trendy Queen Street West on Saturday were replaced on Sunday by swift and aggressive police tactics to shut down protests.

After a day of carnage that gripped the city and resulted in more than 600 arrests, police were left with little choice but to switch gears, said one security expert.

Police arrested people one-by-one after hemming in a crowd on all four sides in a downtown intersection. An officer had warned journalists that they were getting ready to use tear gas and rubber bullets on the group at Queen Street and Spadina Avenue, but that did not happen.

Passersby who were not taking part in the march, and several journalists, were hemmed in as well.

A seemingly peaceful protest earlier in the day outside a film studio being used as detention centre for G20 protesters was quickly put down by a flood of riot police.

A bike rally across town was similarly set upon by police.

As hot spots flared across the city riot police moved in, again surrounding protesters.

The swift action Sunday was in stark contrast to Saturday, when riot police simply watched as anarchists burned their cruisers. It was a deliberate change in tactics, said Michel Juneau-Katsuya, a security expert based in Ottawa.

"They gave a chance to people to calm down (on Saturday) — it didn't work, so the strategy totally changed," Juneau-Katsuya said.

"It's also a way to send a message to the demonstrators and telling them, 'Hey, enough is enough. We are not going to tolerate this. That's why we're ramping it up.' "

The Integrated Security Unit wouldn't comment on whether police had switched tactics.

"Today was a rather peaceful day," said ISU spokeswoman Jen Gearey. "The protesters were rather peaceful, so that's definitely promising."

The massive police force assembled for the twin summits will remain in the city Monday and will be at the ready, she added.

The tactical change Sunday saw protesters, pedestrians and even journalists searched at the whim of police, especially those dressed in black.

Black Bloc is a violent protest tactic in which anarchists infiltrate large peaceful demonstrations wearing normal street clothing. They then change into black clothing and cover their faces in balaclavas or ski masks before fanning out and destroying symbols of capitalism.

The uniform look makes it difficult for police to identify who smashed windows or spray painted a building. Protesters can change out of the black clothing and go unnoticed in the crowd by police.

The change in mood by police Sunday was so sharp, officers chose even to seize a gas mask carried as by a journalist for protection.

"I was in a cell with joggers who were jogging by and arrested, and they spent 16 hours in there," said Matthew Beatty, an observer who was arrested Saturday night outside a Toronto hotel.

"There's some still in (the detention centre). There's a tourist still in the cage that I was in who was just walking, trying to get to his hotel and they wouldn't let him get out of the block."

Many said they were arrested for breach of peace, but not charged with any crime.

The police are calling it "catch and release," said Alan Tang, 23, who padded out in his stocking feet to the cheering crowd gathered outside the detention centre.

Shortly after noon Sunday about 100 protesters outside the detention centre saw an unmarked van speed up, men in plain clothes snatch one of the protesters and whisk him away.

That raised the ire of the crowd, but the anger had little time to percolate.

A flood of riot police rushed in, smoke devices were fired at the crowd and the protesters immediately fled with police in pursuit.

At the bike rally, officers on bikes moved in on the group of 250 cyclists, who had barely finished making a speech outlining their peaceful intent.

Some cyclists broke away but were met by riot police.

Police arrested four people after witnesses saw two of them emerge from a manhole near the summit security zone in the wee hours of Sunday morning.

A spokeswoman for the Integrated Security Unit said the safety of international leaders at the summit was never at risk. But workers were welding shut more manhole covers that lead to "underground infrastructure" as a precaution.

More than 600 people were under arrest by Sunday evening. Many were already being processed at a special court.

Police said some of those detained were not from Toronto, but had come from other cities and towns, even outside of the country.

Edward Canavan said he was walking along Queen Street as protesters were rioting and burning cars. He happened to see a box of oranges, which he didn't realize belonged to officers, and grabbed one, which got him arrested.

"I was just in the wrong place at the wrong time," said Canavan.

Canavan said he was treated well and offered a sandwich by officers during his detention in a wire cage.

"There was a place to stand in a cage, about six or eight people in a cage."

Bridie Wyrock, 20, from Cleveland, Ohio was dressed in a green track suit, handed out at the jail to anyone who felt cold, as she left. The holding cell was chilly and dank.

"Terrible, they put us in cages, blocked off on all three sides," said Wyrock, who was held for 19 hours, but not charged. There were not enough toilets, and other people inside were resisting arrest, she said.

"It was cold and dirty," she added, but did say the police treated most people with respect.

Earlier Sunday police raided a building on the University of Toronto campus. At least 70 people, not believed to be students, were arrested and charges are pending.

The Integrated Security Unit said officers have found a cache of "street-type weaponry" such as bricks, and bottles with liquid substances.

Dozens of officers were combing bushes and garbage cans, collecting articles of black clothing.

Members of the Montreal-based Anti-Capitalist Convergence were among the 70 arrested, said Dani Royer, a spokeswoman for the activist group. But she brushed off accusations they were found with bricks and bottles of liquid.

"I have no idea about that," she said.

The group planned a news conference for Monday in Montreal to denounce police tactics at the summit.

The core group of militant protesters Saturday, disguised with dark bandannas, and wearing black bike helmets, threw bottles and rocks at riot police as they wove through the city's streets.

The angry rampage, which resulted in Toronto police using tear gas for the first time in their history, went well past dark.

The charges against protesters range from mischief, disguise with intent and breached peace to assault and weapons-related charges, said Const. Michelle Murphy with the ISU.

Some people have been released unconditionally, while others have been let go with a promise to appear in court. Still more however will require bail hearings depending on the severity of their charges.

Saturday's violence came as leaders of the world's G20 nations met behind the steel and concrete barrier that has earned the city the moniker "Fortress Toronto."

The city's police chief said Toronto had never before seen such wanton criminality.

Bill Blair said the goal of Saturday's protesters was to draw police away from the security perimeter of the summit so others could attempt to disrupt the meeting of the world's leaders.