The protest, which attracted tens of thousands of people, had already passed and gone down another street when the four-metre-deep sinkhole opened up late Tuesday afternoon.
Denis Roy, an operations chief with the Montreal fire department, said there were no injuries to any passersby.
Public works officials are attempting to determine the cause of the collapse.
The sinkhole is located not far from McGill University on Sherbrooke Street, one of Montreal's key east-west arteries.
The collapse, which happened around rush hour, added to the headaches of drivers already dealing with detours because of the huge student protest over tuition fee hikes and a special law to limit demonstrations.
Photo of the Montreal sinkhole tweeted by DemetriPro:
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