Montreal police rounded up a few dozen people Thursday evening just as they began marching in an annual May 1 anti-capitalist rally.
Police declared the protest illegal even before it began because organizers had not provided their route beforehand.
They quickly isolated many of the demonstrators.
Roughly 10 people were arrested.
The protest had been set to end at Place Ville-Marie, where many banks and mining companies have their corporate headquarters.
A spokesman for the Montreal Anti-Capitalist Convergence, which organized the protest, said it is important for people to get out in force on May 1.
"The powers that be keep repeating that there's no money, we're in a state of financial crisis, etcetera, etcetera,'' Rich Bonemeal said before the demonstration.
"It's not true, there is money. That money is in the pockets of the bosses, it's in the pockets of the banks.
"They're working for the rich to get richer, basically. We can't let the law enforcement just intimidate us.''
Police made 447 arrests in last year's demonstration.
Those detained under a strict municipal bylaw face fines of $637 for a first offence.
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