A city firefighter slashed in a boozy brawl outside a Brooklyn bar described for a jury yesterday the horror of seeing his best pal getting pummeled by a group of men while blood gushed from his own arm.

“I was gushing. I was bleeding out,” Rosario Cicero said, describing how blood from his forearm arm sprayed a foot in the air each time his heart pulsed.

In often monosyllabic testimony punctuated by grunts of “yep” and “nope,” Cicero told jurors how, before the mayhem erupted outside the Kettle Black bar, he and Port Authority cop Ryan McCarthy downed shots of Jameson’s whiskey and “maybe” 10 pints of beer.

McCarthy suffered a broken jaw in the melee, which prosecutors say started when he came to the aid of an off-duty NYPD officer who had allegedly been whacked in the head with a bottle by John DeCarlo, 24.

“He was pretty much curled up in the fetal position,” Cicero said of his childhood pal. “There was probably like four or five people beating him up.”

Cicero grinned as he rolled up his sleeve and showed off his scar from the April 2010 rumble.

“I lost a lot of blood,” he said.

Decarlo and Dan Golden, 25, are accused of punching McCarthy. while Kevin Crowley, 25, is charged with cutting Cicero. Crowley’s brother, Michael, 22, and friend Peter Jung, 25, are also charged in the beating.