Vancouver police are warning the public after a woman was sexually assaulted late Halloween night in Yaletown. 

The victim, who is in her 30s, was walking down Cambie Street near Expo Boulevard when a man threw her to the ground and assaulted her around 3 a.m. Wednesday.

Her attacker ran off after a passing driver pulled over to check on what was happening.

No suspects have been identified, but Const. Jason Doucette said police believe someone saw the suspect running away eastbound on Expo.

"This person may not even know that an assault had just occurred," Doucette said in a news release. "We are hoping they see this story and contact police immediately."

The suspect is described as a man in his late-20s or early-30s who has short, black hair and a medium build. He was wearing a black hoodie and dark-coloured backpack.

Anyone who was in the area around the time of the attack or who might have information on the crime is asked to contact the Vancouver Police Department's Sex Crimes Unit or Crime Stoppers.