Police are looking for a man with a white mask after a stabbing outside a Calgary bar left another man seriously hurt.

A group of people flagged down police at about 1:30 a.m. Friday on the 1200 block of 1st Street S.W.

Police found a man in his 20s on the sidewalk who had been stabbed several times. He was taken to hospital in serious, but not life-threatening, condition.

The suspect ran along 1st Street to a parked white SUV and then drove east on 13 Avenue S.W., police said.

"This attack took place in a very violent – yet cowardly manner – where the attacker stabbed the victim from behind, in an ambush-style attack, without the victim ever seeing the attacker," said Det. Jason Bobrowich.

Officers are looking for a white man, between 18 and 20 years old with a stocky build and light-coloured hair. He was wearing a white shirt, khaki pants and had a white theatrical mask on the back of his head.