A firefighter needed hospital treatment after he was punched in the face as he tried to extinguish a blaze in Preston.
Crew manager Darren Gregson was hit so hard he needed stitches to his jaw after the attack in an alleyway off Waverley Road at 0250 BST on Sunday.
He said the gang of youths were angry because they did not want the firefighters to put their bonfire out.
"I have been to hundreds of jobs like this, it has never turned this nasty," Mr Gregson said.
He said the four firefighters had to retreat back to their engine because the gang had started to pick up bricks to hurl at them.
"We did not expect it to escalate as it did, we only told them we had to put the fire out," he added.
"As soon as we started to spray the fire with water they turned pretty nasty. I have been in the fire service for 10 years and I have never had this type of abuse."
Despite the assault and having to have stitches he returned to work so he could finish his shift.
Lancashire Police confirmed it was investigating the attack. No arrests have yet been made and officers appealed for information.