Organizers of the Toronto protest said the court's decision is an attack on free speech and sets a dangerous precedent for other social movements in Canada.
The case involves Gabriel Nadeau-Dubois, who became a household name in Quebec during the protests over tuition fees last spring.
A judge ruled that Nadeau-Dubois encouraged students to ignore a Quebec City court injunction while he was doing a television interview in May.
His lawyer pleaded ignorance and argued there was no evidence Nadeau-Dubois was aware of the injunction, but a Quebec Superior Court justice rejected the argument.
One of the protest organizers said the ruling was worrisome.
"It's clear to us that this affects not only Gabriel Nadeau-Dubois, but also spokespeople for future movements," said Xavier Lafrance, a student at York University.
"If we cannot state our (case) publicly... that's a real limitation to our basic civil rights."
Nadeau-Dubois has said he doesn't have the money to finance a legal battle and has asked for financial help.