The melee erupted about 6:15 p.m. Friday, when lifeguards working at the pool were attacked, said Vickie Karp, a Parks Department spokeswoman.
"Lifeguards at McCarren pool were attacked by an unruly crowd and the pool had to close to restore order,” Karp said.
The pool usually closes at 7 p.m. but workers were forced to shut it down about an hour early, Karp said.
Witnesses said one lifeguard who tried to break up the fight was punched in the face.
Parks Department officials said there were no serious injuries and the pool would reopen as usual on Saturday.
Pool-goers said on Twitter that a group of teenagers sparked the fight and assaulted several lifeguards.
"Was just @ mccarren pool to witness a gigantic brawl between lifeguards an hs kids," tweeted @MISTER_AOK. "Full riot w ppl elbow dropping offa lifeguard chairs."
Twitter user @apsmithnyc wrote, "Life guard assaulted, nearly drowned by group of kids; McCarren Pool closes early today. Sadface."
The 37,950-square-foot pool just reopened Thursday following a $50 million renovation.
The pool had been closed for nearly three decades after its condition deteriorated in the early 1980s and it became a hotspot for fights and violence, residents recalled.
Jeanette Acevedo, 29, who has lived her whole life in the WIlliamsburg and Bushwick area, said she had been excited about the pool reopening — but now she's worried that it will just attract more of the same trouble it did years ago.
"Here we are, it's only Day 2," she said outside the pool Friday evening.
Acevedo's sister, Migdalia Acevedo, 48, had planned to take her step-daughter to the pool, until she heard about the fight.
"It did make me think twice about bringing her here," Migdalia Acevedo said.