Police said several smoke bombs were set off at multiple points in Montreal's subway system during morning rush hour Thursday, briefly cutting off service and creating a nightmarish morning commute.
Quebec Premier Jean Charest condemned the attack, saying he hopes the guilty parties are found.
"It's inexplicable," Charest said. "There's no reason to commit acts of intimidation and violence. There's no excuse for this."
Bombs were tossed onto the tracks at three stations along the transit network that connects numerous neighborhoods in Canada's second largest city, sending clouds of smoke billowing through stations at key transfer points, said police. Most service was restored just after 10 a.m. (1500 GMT).
Montreal police spokeswoman Marie-Elaine Ladouceur said officers are hunting for several suspects including one man and three women whose photos they received from witnesses.
Thursday's subway shutdown caused long lineups at bus stops and increased traffic during a rainy morning. Some commuters resorted to hitchhiking along the city's busy boulevards, while others hopped on bicycles for a wet ride to the office.
"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 a cramped station, where people huddled inside.
Subway service has been interrupted in recent weeks as the city deals with student protests over planned tuition increases.
On April 16, someone pulled emergency brakes at five different subway stations in Montreal, shutting down the system temporarily. At about the same time, bags of bricks were thrown on the tracks of three lines.
Police have repeatedly said in recent weeks that some radical groups have been taking advantage of students' anti-tuition battle to create their own damage.
Last month, 85 people were arrested during riotous protests by thousands in Montreal over planned the tuition hikes of 325 Canadian dollars ($324) a year. During those protests, windows of local businesses, banks and cars _ as well as a police station _ were smashed after talks broke off between the provincial government and student groups.