Gary Mattson is shown on a security video moments before he attacked Edmonton bus driver Tom Bregg. (Alberta Justice)Gary Edwin Mattson punched Tom Bregg in the face three times, then stomped on the veteran Edmonton transit driver's face 15 times after dragging him off his bus.

The vicious attack on Dec. 3 was recorded by on-board security video and shown Monday at Mattson's sentencing hearing in Edmonton.

Bregg suffered life-threatening head and facial injuries in the attack, which occurred after Mattson got on his number 10 bus in northeast Edmonton during the morning rush.

Mattson, 25, pleaded guilty earlier this month to aggravated assault and assaulting a peace officer.

At the start of the video, Mattson is seen getting on the bus. He argues with Bregg for about two minutes. There is no audio on the tape, but the court has been told the argument was about bus fare.

Edmonton Transit driver Tom Bregg leaves the courthouse on May 6. (CBC)Mattson doesn't pay for his ride and at one point, the doors on the bus close, the lights dim and Bregg starts driving.

Mattson continues arguing with Bregg and points a finger in his face. Bregg pulls the bus over and that's when Mattson punches him, drags him out of his seat, and repeatedly stomps on his face before fleeing.

After showing the video to the court, Crown prosecutors read a list of Bregg's injuries which required multiple surgeries.

Bregg is still suffering from his injuries. When he was in court on May 6 for the first day of the sentencing hearing, his left eye was shut.

The sentencing hearing will resume on May 25. The Crown has indicated it is seeking to have the court designate Mattson a dangerous offender.