Montreal police briefly shut down St-Denis Street southbound from Cherrier to Sherbrooke streets after a bus went up in flames.
Nobody was hurt in the incident and police say it appears to have been caused by a problem with the engine.
Mathilde Gay was on her way to work at 7:45 a.m. with her husband at the wheel when she noticed the black smoke from several blocks away.
She whipped out her cell phone and began recording.
Gay said she didn't see anybody in the bus as they rolled by.
"The bus was stopped on the side of the road," she said. "As we were driving by, basically, the bus started flaming. There were sparks. It was completely on fire."
The STM bus, line 24, had about 15 to 20 people on it when the driver noticed smoke coming out fo the back of the vehicle at around 7:40 a.m.
The driver quickly pulled over on St-Denis near Rigaud Street and evacuated the bus.
Within an hour, the burning bus was extinguished by Montreal fire fighters, but the damage was extensive.
The cause is unknown at the moment, said an STM spokesperson, but the agency will investigate in co-operation with the Montreal fire department.