A Calgary bus driver who leapt into action when he saw a woman in trouble is being hailed a hero for helping to prevent a sexual assault.

The incident happened Tuesday night when the young woman got off a bus around 11 p.m. in the city’s north end and went into a bus shelter to wait to transfer to another bus. A man in his late teens or early 20s, whom she didn’t know, approached her in the shelter and tried to talk to her.

When she brushed him off, he grabbed her, knocked her repeatedly to the ground, and began to sexually assault her.

Just then, Calgary Transit driver Amandeep Hunjan, drove his bus into the area and saw a scuffle in the bus shelter. He immediately turned his bus to drive over.

The transit driver’s arrival caused the attacker to take off running, which is when Hunjan noticed the woman.

"She told me, ‘The guy punched me in the nose’ and was doing something wrong, like sexually assault her,” he told CTV Calgary.

Hunjan called 911 and waited with her until an ambulance and police showed up.

Calgary Police Acting Staff Sgt. Sean Chartrand says the attacker escaped on foot but they have launched an investigation.

“This is absolutely concerning to us. We have a number of leads that we’re pursuing and we’ll follow those to the fullest,” he said.

Calgary Transit has also provided video captured from the shuttle bus camera.

Hunjan says he’s just thankful he showed up when he did.

“I feel good. At least I saved someone's life, because that's a shameful act for someone to do that,” he said.

Det. Sean Chartrand of the sex crimes unit says the woman is shaken by the attack.

“Given the circumstances, it's a very traumatic incident for anyone to go through,” he said. “Physically, she is doing fine, and we have been in touch with her today.”

With a report from CTV Calgary Alesia Fieldberg