An Ottawa bus driver has allegedly been caught threatening a mentally ill passenger in an expletive-laden rant posted online.

A video uploaded to YouTube on Thursday appears to show an OC Transpo driver shouting at a young man standing near the front of a bus, but the bus driver cannot be seen.

"Don't say another f--king word," a voice can be heard yelling. "If you don't shut your f--king face I'm going to stick my fist in it."

Seconds later, the voice instructs the young man to exit the bus "when I open that f--king door" or face the prospect of getting beat up.

The YouTube user who posted the iPhone footage said the incident took place on OC Transpo Bus 6671 while it was travelling to Kanata from downtown Ottawa.

He spoke to CTV Ottawa by phone Sunday but wished to remain anonymous.

"I honestly thought a crime was going to be committed, so I just thought I would pull out my phone and videotape this," he said.

The witness added that the incident was "pretty bizarre" and "the bus driver was really mad."

In the video's description, the witness wrote that they were alone on the bus with the young man who was reportedly speaking as if he were "the host on a game show."

The young man then walked to the front of the bus to talk to the bus driver who eventually yelled at him, the user "DartPak" wrote.

"After trying to apologize and state many times, ‘I'm sorry, I have a mental illness' the young man was subjected to verbal abuse from the bus driver," the description reads.

The incident could be a PR blow to the Ottawa bus driver's union, ATU 279, which has been dogged by a labour dispute in recent months.

ATU 279 vice president Mike Aldrich said the union "does not condone" the kind of behaviour seen in the video, but wanted to know more about the circumstances leading up to the confrontation.

"I'd like to see what happened at the beginning of this to escalate it this far, because it's pretty hard to push somebody that far," he said.

Ottawa Mayor Jim Watson posted a message to Twitter saying there would be an investigation.

"Regarding OC Transpo video -- like anyone I am very disturbed by what I have seen and heard and want to see a thorough investigation conducted," he wrote.

With a report by CTV Ottawa's Karen Soloman

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