Police quickly released pictures of four suspects after devices were set off at multiple points during rush hour, along the transit network that connects large swaths of the city.
The ensuing scene was like a snapshot of the city's spring: a downtown disrupted, travellers impeded, and seemingly every segment of society debating whom to blame.
Many angry commuters condemned striking students. Student protest leaders pinned it on rogue troublemakers. Some even suggested the fault actually lay with Premier Jean Charest, for not bending to the protesters' demands.
It wasn't the first subway interruption Montreal has experienced in recent weeks, as the city has dealt with some unwieldy student demonstrations.
But this attack was notable for its scope and synchronization.
Police said the devices went off between 7:45 a.m. and 8:30 a.m., sending clouds of smoke billowing through the underground at key transfer points throughout the city. As commuters were ushered out of the system, the transit authority said the smoke had paralyzed every subway line.
One journalist asked the city's mayor whether it was time to call in the Army.
"Certainly not — that's the last thing I could think about," replied Mayor Gerald Tremblay, who added he's doing everything humanly and financially possible to protect Montrealers.
The increasingly exasperated Tremblay had called an impromptu news conference to condemn the attack and make yet another appeal for calm. He urged everyone — the government, student groups, even parents — to find a peaceful solution.
"We deserve better than this," Tremblay said. "There are people who have to start doing something as quickly as possible."
But Thursday's incident underscored the lack of imminent solutions to the unrest triggered by planned tuition increases.
Both sides appear to have dug in their heels.
And even if polls suggest a majority of Quebecers quietly support the tuition hikes, the protest movement has attracted the vocal backing of numerous celebrities and disparate groups including environmentalists, trade unionists, anarchists, communists and hardline sovereigntists.
Student-group leaders distanced themselves from the actions — but in the next breath promised more unrest. Now that student associations have massively rejected what was billed as a deal with the government, the movement is planning more protests including a major one on the 22nd.
Charest, whose government has struggled to find common ground in its negotiations with student unions, said he hoped those guilty in Thursday's attack would be found.
"It's inexplicable," the premier told reporters in Gatineau, Que. "There's no reason to commit acts of intimidation and violence. There's no excuse for this — none."
The latest incident, like some before it, attracted some international news coverage on Thursday.
Police hoped all that media attention would help them identify the suspects. They circulated photos of potential suspects — which police received from witnesses — in the aim of tracking them down.
The department said it was searching for four people — one man and three women, all of whom were believed to be in their 20s.
Police did not immediately blame anyone for the disruption. They have repeatedly said in recent weeks that some radical groups have been taking advantage of students' anti-tuition battle to create their own damage.
The smoke bombs were set off at locations including the Jean-Talon and Lionel-Groulx stations, major transit hubs that each house two separate subway lines.
Above ground, the disruption's impact reverberated in the city's streets, with long lineups at bus stops under a steady drizzle and heavy street traffic.
Packed city buses zoomed past stranded commuters desperate to find a way to get to work, school and appointments.
Some resorted to hitchhiking along at least one busy city boulevard, while others hopped on bicycles for a rain-soaked ride to the office.
There were reports on social media of some incidents, like ambulances struggling to get through traffic.
An attempt to restart the system quickly failed, with another smoke attack reported, prompting authorities to declare yet another shutdown. Service began returning on a gradual basis after 9:30 a.m. and it was fully re-established after 10:40 a.m., but for many the damage was done.
Commuters vented their rage.
One listener wrote to local radio station CJAD to say that Charest should withdraw whatever offer he has made to students, and demand that the Army be called in. Other commenters on news websites likened it to domestic terrorism.
Once subway service resumed, commuters walked briskly through the Place d'Armes station just outside Old Montreal. Some jostled their way through the mobs.
In one incident, a woman rushing through the station bumped into another woman. The victim of the collision responded by pushing the other woman in the back and shouting at her.
But there were also reports of civic-mindedness Thursday, with people offering rides and with a temporary increase in BIXI public-bicycle service.
Many locals also waited calmly for service on the subway to resume.
Some sat quietly on crowded subway platforms, typing on their laptops. Others waited in the rain for lifts, taxis or for the possibility that the system would restart soon.
"I've had to cancel a few things and push things later on in my day, so it kind of makes things frustrating," said Marlene Bambonye, as she took cover from the drizzle outside the cramped Laurier station, where people huddled inside to stay dry.
"But, you know, it's kind of part of the city. So, you get used to it. You make do."
As soon as Bambonye heard that smoke had interrupted service, she immediately thought it could be related to the student strike.
One anti-tuition protester, who took part in a demonstration Thursday, didn't condone the smoke-bomb offensive, but said he wasn't surprised things have degenerated.
Mathieu Boily, a university graduate, said people are frustrated with the Charest government.
He accused the government of attacking protesters with "psychological violence" by not answering their questions or listening to their demands.
"I don't think it's a good idea at all," Boily, 35, said of the smoke bombs, which he believes were "isolated acts."
"But I understand why it's happening because... there are no popular revolts in the history of civilization that didn't have actions."
Also on HuffPost