Protesting students took to Montreal's Metropolitan Highway during rush hour, blocking traffic between the Cavendish and d'Iberville exits.
The students are believed to have been demonstrating earlier in the afternoon at the Montreal offices of Quebec finance minister Raymond Bachand.
Then they walked to Highway 40, and took over the eastbound lanes just after 3 p.m. before police forced them off 15 minutes later.
The incident stopped traffic altogether, aggravating regular rush-hour congestion.
Police were directing drivers off the Met at the Christophe-Colomb exit as a security measure.
Student protesters then headed south on the island, with some of them ending up in front of the Édouard-Montpetit metro station, where they blocked street traffic.
Montreal Police are using Twitter for protest and traffic updates.