A teenager was attacked outside the former Chorlton Leisure Centre.
The 19-year-old was trying to calm down an argument outside the building, on Manchester Road, at around 1.30am on Sunday, August 18.
A party was going on inside at the time.
Police said the man was assaulted, possibly with a bottle, and left with severe injuries to his head and face.
He is understood to have been attacked by two men.
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The attackers are believed to have gone inside the building following the incident.
Police said both men were white, in their 20s, and had long, dark hair.
Anyone with information is asked to call police on 0161 856 9979, quoting incident number 383 of August 18; or Crimestoppers, anonymously, on 0800 555 111.
