Hundreds of demonstrators besieged Montreal's Grand Prix street parties Friday night, as police used pepper-spray and reportedly rubber bullets, arrested a dozen people and forcefully pushed back crowds of protesters and bystanders alike.

Starting at around 9 p.m. ET, a group of about 200 demonstrators formed on downtown Crescent Street, right next to a street festival for Grand Prix revellers. A protest march of another several hundred people headed toward the same location.

Montreal police and officers from the Sûreté du Québec, the provincial force, spent two hours in a cat-and-mouse game trying to block the second group's approach to the area try to prevent a confrontation between demonstrators and Formula One fans.

Eventually, the two crowds of demonstrators bracketed the party, one group at the foot of the Crescent Street zone and one at its head. Baton-wielding riot police formed lines at both ends to prevent the marchers from milling into the bar scene, which made for some bizarre moments.

Davide Gentile, at the scene reporting for CBC's French-language news channel RDI, called it "an extremely delicate situation" as hundreds of F1 revellers partied within eyeshot of protesters. At one point, all that separated them was a single line shoulder-to-shoulder police.

Montreal's 46th straight nightly demonstration, comprising people supporting striking students and others opposed to Quebec's Bill 78, heads toward the Grand Prix party area downtown. (CBC)

Gentile said police were undertaking the difficult task of trying to sort out protesters from Grand Prix fans to let the latter into the street festival. But that often proved futile, and protesters penetrated into the bar-and-club areas, prompting police "to try to subdivide the knot of protesters and F1 fans," Gentilly said.

A few of the hundreds of officers patrolling downtown Montreal on Friday night were carrying weapons that launch rubber bullets and tear gas. Media reports said some rubber bullets were fired, including on a man who was hit by one in the thigh after throwing a rock, but police would not confirm anything as of midnight, citing the need for operational confidentiality.

Police did use pepper-spray at various times.

Montreal police said 12 people were arrested on allegations of Criminal Code and municipal bylaw violations. Some of those detained were dressed in the fancier apparel of the F1 party crowd and did not appear to be protesters at all. Those arrests might have stemmed from isolated, angry confrontations between bar-goers and demonstrators.

Through it all, traffic in the downtown core was thrown into chaos, as cars became marooned between teams of police charging at protesters to try to clear out intersections.

At one point, a group of bystanders on Ste. Catherine Street was abruptly repelled by police in their attempt to clear the street. At other times, officers shoved people forcefully to move the crowds away from sensitive areas.

F1 opposition

For weeks, protesters have been promising to disrupt the Montreal Formula One Grand Prix, one of the biggest annual tourist events in Canada. Race-week festivities typically include thousands of wealthy tourists who pay hundreds and even thousands of dollars for tickets to the F1, and also spend time sipping cocktails amid parked Lamborghinis on Crescent and Peel Street.

Some of the protesters also say they oppose how the Grand Prix exploits the sexualization of women.

Teams of police and security guards spent Friday afternoon and evening patrolling the public Grand Prix street parties in downtown Montreal, blocking people wearing red squares — the symbol of the province's ongoing student uprising — from entering and ferreting them out if they slipped by.

Grand Prix weekend organizers have increased security measures to counter possible clashes with protesters.

Police apprehend a protester in downtown Montreal on Friday. Security measures around this year's Canadian Grand Prix events are unusually tight. (CBC)

In one case caught on TV, a high-school-aged teen waving a red Che Guevara flag while walking down de Maisonneuve, which is not closed off for Grand Prix events, was accosted by security guards and made to leave the area.

Protests on Thursday heightened organizers' concerns after students attempted to crash two separate Grand Prix celebrations, on Notre-Dame Street Ouest and on Crescent.

The Société des Transports de Montréal, which operates the city's public transit, has also doubled its security inside the metro for the weekend. Most spectators take the metro to get to the F1 race site on Île Notre-Dame, where dozens of police were deployed Friday, including a canine unit, to keep things under control.

Student protestors have been taking to social media platforms to plan disruptions on the metro system on Sunday, which is race day.

"We work very closely with the police service who are in charge of the security in the metro but we have been preparing all these days with them and are still working with them for the weekend," said Marianne Rouette, a spokesperson for the transit agency.

Tens of thousands of Formula One racing fans headed to the Gilles Villeneuve track on Île Notre-Dame on Friday to watch the drivers' practice runs.

Protesters pledged to target Grand Prix weekend

The Grand Prix race usually attracts 300,000 people to Montreal. CLAC, an anti-capitalist group, has promised that over the weekend it will repeatedly target Crescent Street, which is traditionally the most active bar and restaurant strip during race week.

"Nightly protests will disrupt this crass elite at play in [the west part of] downtown every night," the CLAC group said on its website.

Marc-André Cyr, a historian of social movements and columnist for Montreal's Voir weekly, said CLAC's targeting of the Grand Prix is part of its campaign to disturb society's wealthy classes.

"They would say they're resisting the imposition on people of the Grand Prix, a 'turbo-capitalist festival,' as they'd call it," Cyr told CBC's French-language news channel RDI. "They would say these kinds of events are part of the cult of wealth that are the opposite of democracy."

This year's F1 event has also become a flashpoint in Quebec's nearly four-month-long student uprising, which began as a battle over tuition fees but has evolved into a broader ideological and social struggle.

One-third of the province's post-secondary students have walked out on their classes and Montreal has been the site of daily, occasionally turbulent, street demonstrations.

With files from The Canadian Press