Couillard called the ransacking at Monday night's council meeting dishonourable and appealed for calm as the protests head to the legislature Wednesday.
The premier was equally critical of Montreal police, whose officers appeared to do nothing as demonstrators over-ran city hall and the council chamber itself, tossing papers and plastering the building with stickers.
No amount of anger or disagreement can justify such behaviour, Couillard said Tuesday.
"We cannot accept as a society and I cannot accept as leader of the Government of Quebec what happened yesterday," Couillard said.
Tensions have been rising as municipal employees, including police officers, firefighters and other city workers, demonstrate across Quebec over a provincial proposal to overhaul pension plans.
The government wants contributions and future pension deficits to be split 50-50 between municipalities and unionized workers, a proposal that would entail reworking 170 collective agreements affecting about 22,000 employees.
Hearings into the legislation begin on Wednesday, with a huge demonstration planned outside the national assembly in Quebec City.
Montreal police Chief Marc Parent promised a full investigation into Monday's events and said criminal charges against protesters and disciplinary action for police officers is possible.
Parent reminded his officers they are required to maintain peace and security regardless of their personal opinions and that there are not "two classes of protesters."
But Couillard suggested an impression of such a double standard is on the mark.
"Not only the demonstration itself in the midst of a municipal council meeting but the attitude of the police force to remain passive while other people have the impression — which I think is justified — that (with) different types of demonstrations, the treatment is different," Couillard said.
Police officers often cracked down quickly on students during their numerous demonstrations in 2012.
Monday's protest featured municipal workers storming the main chamber and unfurling a sign calling Mayor Denis Coderre a thief.
One councillor told of being struck while others said they were pelted with water. Coderre recounted being followed as he took refuge in his office. The demonstrators left of their own accord and no one was arrested.
Montreal's police brotherhood said in a statement a full investigation is necessary — one that includes what orders police brass gave — before officers are accused of being lax or complacent.
Parent insisted Tuesday not all police officers can be lumped together and said video will show some officers did try to act.
"Unfortunate incidents have shaken Montrealers' sense of security," the chief said.
"For a lot of people, the events that went on during the city council meeting have raised a doubt regarding the SPVM (police) officers' ability to do their job in a total neutrality and with the professionalism rightfully expected by the population."
Coderre denounced the protest as intimidation and bullying and said there will be consequences.
"They (protesters) violated city hall last night, they violated democracy," he told reporters, reiterating his confidence in the police and Parent.
"They didn't attack Denis Coderre last night, they attacked Montreal and Montrealers through city hall."
The Liberal government introduced Bill 3, its proposal to overhaul municipal pensions, in mid-June, saying the plans carry a collective deficit of about $3.9 billion and aren't sustainable in the long term.
Unions dispute those figures and say it's a cash grab by municipalities looking for labour cost savings and that the government isn't willing to negotiate.
Couillard said he hasn't heard any constructive suggestions from unions thus far, just loud protests.
He is hoping union members act responsibly.
"I've been somewhat disappointed by the lack of frank condemnation from certain union leaders vis-a-vis what happened (in Montreal) yesterday," Couillard said.
In the aftermath, various union leaders did weigh in. The Montreal firefighters' union issued a statement appealing for calm from its members after earlier defending their frustration.
In Quebec City, union leaders from outside Montreal told a news conference that incidents like Monday's only serve to undermine their cause.
Denis Marcoux, head of the federation of public service employees, called the incident "unacceptable" but warned of mounting frustration among his members.