Another 57 employees have been suspended without pay for periods ranging from one week to six months, executive committee chairman Pierre Desrochers said Thursday.
The workers were angry about a provincial government proposal to change their pensions so that all public-sector workers contribute a higher percentage to their plans.
About 100 public-sector workers took part in the demonstration that saw a large group burst into city hall on Aug. 18.
Local politicians were forced to flee from their regular session as protesters threw documents and other objects from their desks while rampaging through the chamber.
Montreal police, who were present for the demonstration, were criticized for not intervening.
Ronald Martin, head of the firefighters' union, called for calm but insisted the firings would be challenged.
"Our members are shocked and disappointed," he told a late afternoon news conference.
Desrochers said four white-collar employees, seven blue-collar workers and 46 firefighters were those suspended.
"There were some who even attacked the mayor's office, others who broke windows," he said. "It's a question of the act's gravity. In certain cases, it was really serious . . . . It was sheer intimidation."
The sanctions followed an investigation by the city's human resources department and the controller general, who analyzed video evidence in each case. Other employees are still under investigation.
Desrochers said all the employees had the chance to defend themselves but not all of them took the opportunity. He said he was aware the sanctions would be challenged.
"We can expect them to be unhappy but we also expect that the vast majority will understand that serious acts were committed that are unprecedented.
"These types of acts absolutely cannot be accepted by the administration. "They (the employees) must acknowledge their responsibility."
In Quebec City, Municipal Affairs Minister Pierre Moreau supported the city's decision.
"The authorities will not tolerate illegal acts," he said. "We are in a democratic society, we can accept that people disagree with our positions but we cannot accept it when illegal acts are committed."
The sanctions will not affect police officers present who did nothing to stop the protest.
Desrochers said Montreal police are conducting their own internal investigation into that.
As well, 44 city employees were arraigned Thursday on various charges, including unlawful assembly, mischief and assault, in connection with the city hall protest.
— With a file from Canadian Press reporter Julien Arsenault