A heroic off-duty cop was slashed yesterday in a fierce struggle with a vicious punk who was holding up a Brooklyn Dunkin' Donuts (search), cops said.

Officer Vincent Schiavelli, 24, who has made 64 collars in his two years in the NYPD, walked into the shop just as the heist was going down, cops said.

Dramatic surveillance photos show the robber — who wore a Yankee cap and was later identified by cops as 22-year-old Shron Killings — jumped on top of the counter and lunged at the clerk, trying to slash him with a folding knife.

The robber was reaching into the cash register when Schiavelli — wearing a white T-shirt — grabbed him from behind and the two began struggling. The punk, a suspect in two other Dunkin' Donuts robberies with two prior arrests, lunged at the hero cop with his knife but missed.

Schiavelli then got the thug in a bear hug and tried to subdue him. That's when Killings slashed the hero cop in the left side and ran outside, police said. He drove off in a red Kia SUV and was being sought yesterday.

Schiavelli, unaware of his wounds, ran outside and gave chase, but collapsed in pain. He rushed back into the store and called for help.

Schiavelli, who suffered a cut about an inch long where the chest meets the abdomen, was taken to Kings County Hospital, where he was in stable condition. He was kept overnight for observation.

Schiavelli was visited by his parents, Paul and Kathy, and by a steady stream of fellow officers.

"I've been proud of my son since the day he was born," said Kathy as she left the hospital.

Police Commissioner Ray Kelly was effusive in his praise of the brave officer. Schiavelli "did a terrific job. We're proud of him. He took immediate action without concern for his own safety," Kelly said.

His actions "represent the finest traditions of the NYPD. If he wasn't there, the clerk may very well have been wounded."

Mayor Bloomberg said, "Although we have come to expect this kind of heroism from our police officers, what makes Officer Schiavelli's actions so exceptional is that he was off duty at the time.

"This young officer, out of uniform and without a partner, didn't hesitate to put himself in harm's way to protect a fellow New Yorker."

The drama at the store at 40 Empire Blvd. in the Prospect Lefferts Gardens (search) section unfolded at 6:40 a.m. as Schiavelli was heading to work at the nearby 71st Precinct in Crown Heights (search).

The surveillance video showed the robber walking into the shop carrying a dollar in his left hand and ordering a French cruller. No one else was there.

When the clerk turned to get the doughnut, Killings took the folding knife out of his right pocket but held it under the counter.

He gave the dollar to the clerk, who opened the register. At that point, he lunged at the clerk and Schiavelli grabbed him.

As the suspect fled, a witness copied the license plate of the SUV and cops traced it to Killings' mother, who said she lent it to her son.

Cops said Killings was a suspect in a $300 stickup at the same store on May 17 and a $400 holdup at a Dunkin' Donuts on Utica Avenue on May 25.

An accomplice, Jims Medy, 18, was arrested on June 11 in connection with the first robbery.

Killings' prior arrests are for criminal possession of a weapon and unlicensed operation of a motor vehicle.

Shalha Khairkhah, the manager of the Prospect Lefferts Gardens shop, said: "We were very lucky the officer was there. Otherwise I don't know what would have happened."

Demetrius Hinson, 23, who works at a Wendy's next door to the doughnut shop, said he moved to Brooklyn two weeks ago from Charlotte, N.C.

"This is all shocking to me," he said. "This doesn't make me want to stay in New York City. It makes me want to grab my things and get out."