Clashes erupted when thousands of Quebec students stripped to protest against a proposed tuition hike in a bid to embarrass the hosts of the Formula One Canadian Grand Prix here in Montreal.
The protests and media attention have already put a dent in the global sporting event, which usually generates an estimated CAN$100-million (US$97-million) in revenue for the city but this year has seen lagging ticket sales.
Riot police fired stun grenades and tear gas late on Thursday, trying to prevent the protesters from approaching the event, as the demonstrators sought to draw international attention to their three-month campaign.
Nearly 40 people had been arrested by late on Thursday, according to police.
Earlier in the day, hundreds of student marchers stripped down to their underwear, the women thinly concealing their breasts with red body paint and the red cloth squares that have become a symbol of the protests.
"Everyone naked in the street," they chanted as they banged pots and pans together — another feature of the nightly demonstrations held across the mostly French-speaking province.
"Formula One! Polluter! Sexist! Thief!" they shouted.
Demonstrators said they had removed their clothes in order to demand more transparency from the province, which is partly funding the race while also demanding that students pay a larger share for university education.
"Our nudity expresses the demand for transparency," a male student, clad only in gray cotton underwear, told AFP. "The government is masked. It conceals its own motives, as we have seen in the negotiations."
The student requested that his name not be printed for fear that his mother would find out he was taking part in the protests.
Since February, hundreds of protesters have been arrested and clashes have erupted sporadically as more than 165 000 students have refused to attend class and tens of thousands have taken part in nightly demonstrations.
Quebec Premier Jean Charest, whose popularity has plummeted over the crisis, had earlier expressed anger at the direct threats against the Grand Prix.
"When we attack the Grand Prix, it's not the Quebec government that people are assaulting, it's all Quebecers," Charest said.
Students last week rejected a government offer to reduce the tuition hike by Can$35 ($34) per year, which would bring the total increase in annual tuition to Can$1533 ($1473) over seven years instead of Can$1778 ($1708).