A 17-year-old boy suffered a fractured skull after being punched to the ground in Blackpool town centre.
The teen was taken to Royal Preston Hospital, which was put on standby for his arrival and specialises in serious head injuries, after being knocked out in the attack.
No arrests have been made, police said, and officers are investigating the incident, which happened in Church Street at around 12.30am on Sunday.
One witness, who was working nearby, said there had been a 'big argument between two big groups of people'.
"Suddenly, one of the lads took off his shirt, ran up to another lad and punched him so hard he was knocked unconscious in the middle of the road," he said.
He said the attacker then hit the teen again, 'in his head with his fist or knee', and added: "There was a big panic and they moved him onto the path and the other lad ran off."
An ambulance was called to the scene at 12.30am and arrived nine minutes later, a spokeswoman for the North West Ambulance Service said.
She said the victim had 'facial injuries', while the witness said: "The ambulance was still outside with him in the back an hour later.
"The blood is still on the pavement as of last night."
Witnesses have been asked to call 101 with information, quoting log number 0045 of Sunday, July 1.