Police declared the protest illegal as a crowd assembled at Place Émilie-Gamelin near the Berri-UQÀM metro station late in the afternoon.
Officers quickly surrounded the demonstrators who began to march around the square shortly after 6 p.m.
CBC's Alex Leduc said several journalists from other media outlets seemed to be caught up in the crowd as officers carried out kettling manoevers.
Montreal police said three people were arrested for assault.
The demonstrators were speaking out against municipal bylaw P-6, which allows police to declare protests illegal if organizers fail to provide authorities with an itinerary.
The bylaw also makes it illegal for people to cover their faces while taking part in protests.
Police ushered the people who were arrested onto city buses, where most of them were processed and eventually released.
No injuries were reported.