Police are searching for a man believed to have sexually assaulted a 28-year-old woman at gunpoint overnight.

The victim was walking downtown near Alexander and Yonge streets at around midnight Saturday when the suspect approached her and pulled out a gun, Toronto police said.

He then took the woman behind a building, sexually assaulted her and ran off, according to police.

The suspect is described as a black man with a medium complexion, who stands at about five-foot-10 and has a medium build.

He looked to be between 25 and 30 and wore dark clothing, police said in a statement.

The incident was originally described as a robbery in a tweet from Toronto police.

Police are asking that anyone with information contact them at 416-808-7474 or through Crime Stoppers at 416-222-8477.