Police are looking for both the assailant, and the passerby who intervened, in a sexual assault in the downtown core. 

Investigators say a 25-year-old woman was grabbed from behind and pulled into a laneway near Ryerson Avenue and Queen Street West just before midnight on Saturday.

'Witnesses sometimes don't think to stay around.' - Const. Craig Brister

The assailant knocked her down and sexually assaulted her, but the attack was interrupted when someone shouted at the man to leave her alone.

"Our suspect then got up and fled west through this alley," Const. Craig Brister told reporters at the scene of the crime on Sunday. The alley, known as Linda Griffiths Laneway, runs west from Ryerson Avenue. 

It's not clear who shouted at the attacker. Brister said it could have been someone on the street, or in a nearby home. The victim, who suffered minor injuries, called police herself. 

"Witnesses sometimes don't think to stay around, they don't want to stay around, they don't want to get involved," he said. 

Police are asking the person to come forward, either by contacting the sex crimes unit at 416-808-7474 or Crime Stoppers at 416-222-8477. 

The assailant is described as a white man between 30 and 40 years old, standing five feet, 11 inches tall. He has a chubby build — approximately 250 pounds — with short, wavy black or brown hair. He is unshaven, has a large nose and was last seen wearing a dark hoodie, dark grey track pants and dark shoes.