Donna Coyle, from Creggan, was in town with her sister last Saturday night to celebrate her birthday when they witnessed the attack on Terence Cassidy.

Watching in horror, they saw the teenager punched from behind by an unknown man who then made a sick comment before walking away.

Terence, from Hollymount, woke up in hospital last week with a fractured jaw, skull and no memory of his attack. The teenager said he had been at a party on Saturday night and decided to go into town to see a couple of friends. He spent four days in hospital with serious injuries and bleeding from his ears, and doctors say he could yet lose the hearing in his left ear.

Speaking of the night they found Terence, Donna said: "I just saw (the attacker] coming out of nowhere and hitting the fella and he fell to the floor like an ironing board. He was bleeding from the ears and we put him in the recovery position and called for an ambulance. I have two sons and to get a call like that would scare the living heart out of you. To think of something like this happening to them just doesn't bear thinking about. The main thing is that he is OK now."

Donna said Terence's mum, Olive Quigley, thanked her for finding her son and getting him the help he needed on time. She also made an appeal for people out on the town on a weekend night to keep their eyes open and watch out for people in need of help.

She said: "If you do see anyone lying on the ground, don't just think they are drunk, go and have a look and make sure they are alright."

Police are still looking for Terence's attacker who is described as being 5'9, in his early 20s with a slim build and dark hair. He was wearing a white tracksuit top and jeans and made off towards Waterloo Place.