MONTREAL—The calm of summer is being shattered with the return to school of Quebec universities, where some classes are being disrupted as protesters disobey the back-to-school law.

Chaotic scenes, which are reminiscent of those seen across the province in the spring, have suddenly flared up this morning after a relatively quiet summer.

Masked protesters at the Universite de Montreal warned a news photographer that he had better not try taking pictures of them: "Be careful," she told him. "They're going to take care of you."

The provincial emergency law, known as Bill 78, sets stiff fines for people who block classrooms.

The vast majority of Quebec's students have voted to end their strikes, and the student unrest has hardly been an issue in the current provincial election campaign.

But today's events, as universities reopen, are a flashback to events that captured international attention last spring.

The return to CEGEP had occurred with tranquility in recent days but today's reopening of universities saw showdowns at the Universite de Montreal and the Universite du Quebec a Montreal.

At the UdeM there was a showdown between security and a few dozen protesters, mostly wearing masks. Lines of guards were protecting a hallway where some protesters had tried to disrupt a class.

In front of them, meanwhile, the crowd of protesters was holding votes about whether to charge the line of guards. Local riot police arrived in the hallway to clear the area. The protesters fled, tossing objects behind them.

A fire alarm was soon pulled, its sound wailing even into classrooms in departments not involved in the conflict.

At UQAM, small groups of masked protesters, armed with lists, were seeking out classrooms in faculties where students voted to remain on strike. They were interrupting the classes by shouting and shutting off the lights. Some of the classes were cancelled.

“We have the list of exactly which classes to block and we want to make sure to respect the students who voted against the strike,” said UQAM student leader Alexandre Poulin. “We are making sure that no class that is supposed to be held is being blocked.”

Several professors attested to being in a difficult position.

One professor, who teaches students from departments that are on strike and others that aren't, summed up his dilemma.

"I was hoping that either everyone would show up, or no one would show up -- so I wouldn't have to make the decision whether to teach or not," said one professor, who declined to provide his name.

"If three students showed up, should I still have the class?"

As it turned out, when masked students barged into his class shortly after 9 a.m., he was there by himself and the class had been cancelled.

“Ten minutes in, they came into the class, they barged in, there was maybe a dozen of them or so, started making noise, flashing the lights, they had horns, pots and pans, they were disruptive to the point that the teacher couldn’t speak. So they pretty much told us to leave or they were going to stay and make noise,” said one student who didn’t want to provide her name.

In one psychology class at UQAM, a masked protester attempted to convince another student of the democratic legitimacy of their disruption. The actions, he explained, came after legitimate strike votes were held by student associations.

"We're only disrupting classes in departments that voted to strike," he said.

But the student simply turned her back on the protester.

The striking students continued to bang on desks and blew an air horn until the professor raised his hands in frustration and cancelled the class.

Several striking students carried a flyer that listed the departments that voted to continue their strike.

On the back of the flyer there are spots for taking down names, the programs and email addresses of students who defy picket lines.

—with files from The Canadian Press.