Hundreds of protesters spilled into downtown Montreal on Thursday night, many of them naked or barely clothed, as police used stun grenades and truncheons to prevent them from confronting Formula One parties.

Unlike the last week or so of the city's regular nightly demonstrations, which police largely left to their own devices, Thursday's protests were repelled by riot officers whenever they neared sensitive areas.

Those zones included Crescent Street and Peel Street, where fans of this weekend's Canadian Grand Prix auto race were sipping cocktails amid parked Lamborghinis, and in the city's St. Henri neighbourhood, where about 300 protesters attempted to crash a Formula One gala for race teams and officials.

There were brief clashes between some of the demonstrators and police. Pepper spray was used on a crowd of protesters as some of them tried to enter a public party on Peel Street. Later in the same area, officers set off a flashbang to disperse a crowd. Tear gas was also reportedly used by police.

Throughout the evening, phalanxes of truncheon-wielding police chased groups of protesters through downtown streets, shoving those who didn't move fast enough, striking a few with their batons and arresting a handful of others.

The tumult began at around 6 p.m., as a group of about 300 protesters moved toward the F1 party for race teams and officials.

Several protesters gathered at the St. Henri site donned masks, violating a recently adopted bylaw forbidding face coverings at demonstrations. The bylaw outlaws an entire demonstration if even a few participants are masked, and Montreal police moved swiftly to kettle the crowd.

Police moved through the crowd of boxed-in protesters to conduct searches. They confiscated a few light bulbs filled with paint and several people were arrested. "Why are we being held here? Did we do anything wrong?" one woman yelled at police, who did not reply.

Montreal police spokesman Cmdr. Ian Lafrenière said constables identified sources of potential danger among the protesters and decided to move in and "temporarily detain" everyone about 100 metres from the gala cocktail party.

Officers arrested some of the protesters before the rest were allowed to move on after being trapped for an hour.

Reporters at the demonstration were warned they would be arrested if they remained with the protesters inside the kettle.

Naked protest

A later protest that evening saw about 700 people marching downtown, chanting dans la rue, avec nous — "in the streets with us," a slogan that, intentionally or not, played on the French word nu for "naked." The event was informally labelled a manufestation, another pun on the French words for naked and the word for protest (manifestation).

A large contingent of the crowd wore just underwear or was nude. Quebec news channel LCN even interviewed three naked men on live TV, with their private parts in full frame.

A few of those protesters also sported red niqabs. One of them was otherwise naked but covered in red body paint, while another wore underpants.

About 700 people participated in a march mostly consisting of naked or barely clothed people Thursday evening in Montreal. For some, it was a way to call attention to what they see as the exploitative sexualization of women in Canadian Grand Prix events. (CBC)

Any time the crowd approached Crescent Street or Peel Street, the heart of the Grand Prix celebrations, squads of riot police would run full out to the nearest intersection to cut them off. That prompted some demonstrators to complain that police were using repression to "protect the economic elite."

A third wave of protest began Thursday night, as the 45th straight daily protest march gathered east of downtown, then marched west toward the Grand Prix area.

The several hundred people were blocked from getting near Peel Street, however, by police. At one downtown intersection, police charged into the crowd waving their truncheons, shoving some demonstrators back and frog-marching others off the street. One man was pepper-sprayed by a bike constable from about 20 centimetres away.

On Sherbrooke Street, police also chased crowds back, and two people were arrested near McGill University. One person suffered minor injuries in the stampede of fleeing protesters.

Protesters pledged to target Grand Prix weekend

Thirty-nine people were arrested in the day's skirmishes for breach of the peace and allegations of a variety of offences, including assault. Police spokesman Lafrenière said a few demonstrators threw things at officers and one had a Molotov cocktail.

For weeks, protesters have been promising to disrupt the Montreal Formula One Grand Prix, one of the biggest annual tourist events in Canada.

The Grand Prix race usually attracts 300,000 people, including hundreds of wealthy tourists who pay up to several thousand dollars for tickets to the event.

People flee as riot police charge a crowd. (CBC)

Among the guests at the F1 weekend event are Grand Prix boss Bernie Ecclestone and Michael Fortier, his co-president for the event. Fortier, a Montreal businessman, is a former senator and minister in Prime Minister Stephen Harper's cabinet.

He said the protest at the F1 gala was "targeting the wrong people" because it was raising funds for hospitals. "These people are sharing the wealth," Fortier said.

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