MONTREAL - After a marathon processing session, the 518 protesters arrested downtown Thursday were released in the morning, many slapped with $634 fines for violating municipal bylaw P-6.

As could be expected, those detained in the mass arrested at St-Denis and Sherbrooke near midnight were none too thrilled by the fate.

Those arrested include Henry Gass, a student journalist at McGill who was initially allowed to leave the circled group after showing a press pass.

"At that point I ran away up the alley and got through to Sherbrooke and then ran into more police who were there with the kettle and they arrested me," said Gass.

His wrists were tie-wrapped but he noted that the mood among those arrested remained calm.

The kettling tactic, which sees authorities circle groups of protesters, was used at the G20 protests in Toronto under different circumstances. The tactic has been criticized because it can lead to the arrest of innocent bystanders.

Montreal police said that the measure was required because the group contained individuals pelting unprotected police officers with rocks and other dangerous projectiles after their route down St-Denis was blocked from Sherbrooke.

Those detained were given a bus ride back downtown after they were released following several hours of waiting at the force's East End operation centre

Montreal police representative Ian Lafreniere said that the maneuver was necessary.

"We're not trying to arrest a lot of people," he said. "We don't want to break a record with this but citizens of Montreal asked us to restore the peace, to bring back the Montreal they know and that's why yesterday we declared an illegal assembly at 8:30 p.m. and people were apprehended at midnight."

"So people who said it was impossible for them to leave, three and a half hours later, even if you're not a fast walker, normally you should leave," he continued.

Some protesters said that they were not aware of the orders to disperse or that the protest had been declared illegal.

"I have a hard time believing they were not aware," said Lafreniere.

Gass said he knew of the police announcement before his arrest.

"I don't know if everyone knew, but I know they announced it at the beginning of the march and tweeted it," he said.

The fines could require a long time for the local legal system to digest.

One local lawyer said that those contesting the fines might have to be very sharp in court to win their case.

"The legal question is quite simple: were you taking place in an illegal demonstration?" asked Steven Slimovitch. "Then the legal framework is simple, it's a factual question: what were you doing there?"

"If you were standing there and you were given repeated warnings to leave and you were told by police that this particular situation has been deemed to be a riot, well you've been given a warning," he continued.

"The riot act, so to speak, has been read. It's not an answer to say, ‘I was only here,' because you've been told to disperse."

Behind the scenes of the detentions

In the Montreal kettling scene, a few demonstrators reacted angrily while others sat dazed. There have been reports of tourists stunned to find themselves stuck in the crowd.

The police swiftly squeezed the mob together tighter and tighter. Officers advanced and some people begged to be let out, pleading that they were just walking by.

One photographer was seen pushed to the ground and a piece of equipment was heard breaking. Some protesters cursed and yelled at provincial police officers, who ignored the taunts.

An independent filmmaker, Emmanuel Hessler, had been following the march for a few blocks. He said in a telephone interview with The Canadian Press from inside the police encirclement that he was surprised by the action.

"Suddenly, there were police all around us," he said.

People were carted off onto city buses, which have been used as makeshift police holding pens. The bus drivers' union opposes the practice and wants to stop it.

The crowd waited to be handcuffed and led away to the buses, one by one, to be sent for processing at a police operational centre -- a procedure expected to take several hours.

Meanwhile, a man started reading poetry and the crowd hushed to listen.

Someone else sang a folk song. At one point a woman called out the phone number of a lawyer which the mob took up as a chant.

Hessler, 30, was able to tweet to friends: "We are about to get cuffed and off in a bus. Don't know what happens after. Wish me luck."

The mass arrests came after five days of escalating violence in a dispute that began over tuition fees, evolved partly into a struggle against capitalist practices, and in recent days has mushroomed thanks to opposition to the Charest government's Bill 78.

That bill has not been invoked in any Montreal arrests -- although it has been used elsewhere in Quebec and Montreal police say it could still be used to arrest some protest organizers.

There has been some violence every night of the long weekend and in the first part of this week.

Wednesday night's demonstration looked as though it would break the pattern.

A festival-like atmosphere kicked off in many neighbourhoods as people marched and banged pots and pans in different parts of the city. The percussion-heavy protests have been happening every night at 8 p.m. in Montreal and each night it's become bigger and louder and lasted longer.

The noisy cacerolazo tradition of pot-banging originated as a protest tactic in Chile under successive governments, before and during the infamous Pinochet regime, and it has spread to other countries as a method of showing popular defiance.

One woman joked on Twitter she had broken two wooden spoons and would be bringing a metal one to future pots-and-pans protests.

Wednesday night, there were suddenly thousands doing it in Montreal and they spilled out from their houses into the streets in different pockets of the city in crowds that included children, their parents, students and elderly people.

"It's symbolic because it comes from the revolution in Chile, where there was a dictator in place and there was long tradition of protesting," Sebastien Barraud, a union representative in the education sector, said in reference to Pinochet.

"It also shows that peaceful civil disobedience works. And we're in the process of showing that the polls have no value. A majority of Quebecers are against this government."

Originally from France, Barraud said he also benefited from cheap university fees.

"So I know it's possible. It's just a matter of a political choice."

More than 2,500 people have been arrested in a months-long dispute that has catapulted the province onto international news pages, which is at least five times the number jailed during the 1970 FLQ crisis that saw martial law declared in Quebec.

With files from The Canadian Press