Four people were treated in hospital and half a dozen others got help at the scene after pepper spray was released inside the downtown Montreal bus terminal at Berri-UQAM.
The terminal was evacuated around 10:45 a.m. on Friday, sending about 100 people onto the sidewalk on Berri Street near De Maisonneuve Boulevard.
"It started with a woman screaming," said one passenger. He said he witnessed the entire event while waiting in line for his bus.
He said two guards ran after a male suspect, who he said then sprayed them directly in the eyes with the pepper spray.
"It smelled to the point where it pushed you to cough. It's like air got into your lungs to the point it was unbearable," said passenger Annie Drouin, who used her first aid skills to help some of the victims.
"You had a lot of people panicking because they had trouble breathing. Before [pepper spray] gets to your lungs it makes you cough. It's as if you sniffed cayenne pepper," Drouin said.
Firefighters and other emergency responders administered aid to dozens of travellers, helping them to wash out the eye irritant.
Some experienced symptoms consistent with being pepper sprayed, including difficulty breathing, irritated eyes and vomiting.
Police are looking for the suspect and are speaking to witnesses. They are also reviewing surveillance video footage.
The incident caused some travel delays. The bus station reopened early Friday afternoon.