The man who took the video he said shows an OC Transpo driver threatening a passenger said he started filming because he thought a crime was about to happen.

The man, who goes by the YouTube username DartPak, said he took the video on route 96 in Kanata early Thursday morning when there were only a few people on the bus.

He said this passenger was rambling, appeared mentally ill and tried to apologize to the bus driver who repeatedly swore at him and threatened violence.

"If you don't shut your f--king face I'm going to stick my fist in it," someone can be heard yelling.

"I honestly thought a crime was going to be committed, so I just thought I would pull out my phone and videotape this," said the witness, who didn't want his real name used.

He said before he started filming, the bus driver stopped on the Queensway hoping the passenger would get off.

In the end, the passenger ended up leaving at the Eagleson Park and Ride, apologizing as he left.

The vice-president of Ottawa's bus driver union said they don't condone this kind of behaviour and has reminded his union that they're in the public eye where many have smartphones.

"I'd like to see what happened at the beginning of this to escalate this far, because it's pretty hard to push somebody that far," said ATU 279 vice-president Mike Aldrich.

The witness told CTV Ottawa the passenger was talking into his phone like it was a microphone, then talking to people and answering for them.

He said he got the impression this wasn't the first encounter between the driver and the passenger.

"I heard the bus driver say ‘every night it's the same thing with you, just sit down and shut up and take your meds,'" he said. "Just really inappropriate things in my opinion."

Both Ottawa Mayor Jim Watson and transit commission member Shad Qadri said the city will do a thorough investigation.

If there is any action against the driver, it won't be public – discipline is protected by the Privacy Act.

With reports from CTV Ottawa's Karen Soloman and Vanessa Lee

WARNING: This video contains explicit language.