The protesters tried to gather Wednesday outside a private club where politicians and construction executives have been known to meet.
Details of private rendez-vous at the exclusive Club 357C in Old Montreal have emerged from the province's Charbonneau inquiry.
The probe has heard that construction companies created a cartel that drove up the price of public contracts and split the extra profits with the Mafia, corrupt bureaucrats and political parties.
"Let's bring our rage directly to the secret dispensaries of power: the private clubs where political and economic elites plot and conspire against the people," said an earlier statement from the organizing group, the Anti-Capitalist Convergence of Montreal.
"It is in these quiet lounges and luxurious dining rooms that the capitalists draw up the plans of our oppression and our exploitation."
But demonstrators were prevented from taking their message to the site of the club.
Police blocked off the protest, kettled a number of the demonstrators, and detained about 300 of them. Police said they would hand out numerous fines of $500, plus fees, for municipal bylaw violations while a small number might also face criminal charges.
Officers said they stepped in after projectiles had been tossed from the crowd.
Police have shown little tolerance for rowdy protest in Montreal lately and moved to head off events like those seen repeatedly in the city last year.
That response has drawn criticism from civil libertarians and from demonstrators who argue that their right to free expression is being taken away.
The anti-capitalist demonstrators are equally unlikely to be delighted with one of the provincial government's intended solutions to the corruption crisis: freer capitalism.
The government has said it hopes for more competition in the construction industry, through changes in a possible Canada-Europe free trade deal.
It says it expects any agreement to allow greater access to public-works contracts for foreign companies and make it harder for a small circle of domestic firms to control the market.
The mood was far from somber in the crowd, however.
A brass band, which was among the group kettled by police, played on and kept the protesters entertained while they waited to be led away one at a time and loaded into waiting buses. It was unclear how many might receive fines for municipal bylaw violations.
The scene unfolded as people sitting at sidewalk cafes took photos with their phone cameras. The noisy scene did not prevent them from enjoying their outdoor meals.
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