A vicious brawl yesterday shut down the newly renovated pool at McCarren Park in Brooklyn a day after it opened.

An unruly crowd of teens started the melee at about 6:15 p.m. when they attacked lifeguards trying to stop them from doing dangerous back-flips into the water of the much hyped recreation facility, witnesses said.

Parks Department spokeswoman Vickie Karp said the incident occurred less than hour before the Olympic-sized pool on the Greenpoint-Williamsburg border was to close for the night.

Shocked onlookers turned to Twitter with reports of the watery rumble.

“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.”

“Life guard assaulted, nearly drowned by group of kids; McCarren Pool closes early today. Sadface,” tweeted @apsmithnyc.

McCarren Pool reopened Thursday following a $50 million renovation. Mayor Bloomberg was on hand at the gala ribbon-cutting ceremony, saying the pool “has an illustrious past and a bright future.”

After closing in 1984, it remained an empty basin until 2005 when it was briefly turned into a venue for concerts and movies.

The pool — built in 1936 — is expected to reopen as scheduled this morning, Karp said.