Quebec City police said about 20 people were arrested near the site of rival protests by right-wing and anti-fascist groups.
Spokesman David Poitras said the suspects were heading to the protest with the intention of causing violence, but could not confirm whether they identified with any of the protesting groups.
The arrests came after several hundred members of the right-wing groups Storm Alliance and La Meute gathered to march through downtown Quebec City.
Spokesman Dave Treggett said the group is protesting the policies of the provincial Liberal government.
The right-wing march prompted a counter-protest from a number of anti-fascist groups, who began their gathering in front of the National Assembly.
A spokesman said the counter-protest had been organized in response to what he called racist and violent statements by members of the right-wing groups.