Montreal's police earned a reputation during student protests in 2012 as a force willing to resort to tear gas, pepper spray and some muscled arrests to rein in rowdy demonstrators.

On Monday, they gained a different sort of image – that of a force prepared to look the other way when mayhem and vandalism are being caused by fellow city employees.

In scenes replayed repeatedly on Tuesday, police officers stood by impassively while municipal employees stormed into Montreal City Hall, roughing up councillors and throwing glass and papers into the council chambers.

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In this case, the police happen to be part of the same pension dispute as their fellow civic employees.

Officers' apparent willingness to put solidarity before duty was met with widespread condemnation on Tuesday. Many were seen standing on the sidelines with their arms crossed while protesters – firefighters and other employees – surged into City Hall and invaded the council chambers.

Comparisons with the police's assertive behaviour toward students during their tuition-hike protests were inevitable.

"There can't be two types of treatment" toward protesters, Premier Philippe Couillard said after calling a press conference in Quebec City, going on to say that the "silence of our police forces" allowed city council to be disrupted by "threats and intimidation."

The acts "dishonour the very essence of our institutions and diminish the moral authority of our police forces," he said.

Officers' mostly laissez-faire approach Monday allowed the protest to degenerate. Demonstrators entered the council room as elected officials were about to meet; some wore hooded sweatshirts and bandannas over their faces. They swarmed over the chambers, overturned chairs, and threw a blizzard of papers – pages from their contracts – from the public viewing stands, littering the floor.

One councillor says he was punched in the side inside the council room. Mayor Denis Coderre says water glasses were thrown from the viewing area, and a window to his office was shattered after he retreated there with some councillors.

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"This was an attack on democracy," he said, saying some protesters acted "like savages" and some councillors inside his office were scared.

The city police chief, Marc Parent, promised to probe the incident but was met with criticism for handing the job to internal police investigators.

He acknowledged that police behaviour at City Hall would raise public doubts about officers' neutrality and professionalism. "There were shortcomings in the security at City Hall," Chief Parent said.

The raucous scene underscores growing tensions over Quebec's Bill 3, a Liberal bill aimed at getting unions to kick in a larger share of their pension. As protests grow, so does the challenge to the leadership of Mr. Couillard and Mr. Coderre.

Until now, unions' displeasure with the pension changes has mostly taken the form of police officers wearing camouflage pants and firefighters plastering their trucks with stickers.

Judging from the reaction to Monday's events, the behaviour at City Hall isn't likely to win public opinion over to the unions' side.

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Parliamentary hearings into Bill 3 open Wednesday in Quebec City. While Mr. Couillard said he's willing to listen to suggestions about improving the pension bill, he's standing firm on implementing changes and cleaning up in Quebec's public finances. The legislation seeks to have workers and cities evenly split the cost of filling their pension plans' $4-billion deficit.

"Quebec is entering a period of profound and necessary change," he said.