Police officers cornered and isolate protesters at the start of a demonstration before a cocktail party kicking off Canadian Grand Prix festivities.
Police swept in and surrounded the protesters, many of them masked or wearing black, as the group approached a barrier about 100 yards from the party.
About 20 people were arrested and police said they confiscated light bulbs filled with paint.
The Formula One race usually attracts 300,000 people — many of them tourists. Some protesters promised to disrupt Grand Prix-related events — some out of opposition to tuition hikes, others out of opposition to capitalist practices.