UPPER EAST SIDE — A son slashed his father inside an Upper East Side luxury building early Wednesday morning before jumping to his death from a 46th floor window, according to the NYPD.

Mark Citrin, 27, was acting erratically when his father, Niles Citrin, 65, tried to calm him down inside the family home at 200 E. 65th St. at about 6:50 a.m., according to officials and sources. Mark then attacked his father, slashing him on the head and arm before barricading himself inside of his bedroom, police and sources said.

NYPD hostage negotiators tried to talk Mark out of the room, but the son yelled “No!” and then jumped out of the window, according to sources.

Mark also tossed some items out the window before leaping, including a book titled "Modernist Cuisine," sources added.

Maggie McDonald was walking by the building on her way to work when she noticed a sweater floating out of a window.

“I thought, ‘Oh, that’s weird,’ and was thinking somebody was mad at their significant other,” McDonald said.

Then all of a sudden people started screaming.

"I heard this thud that I’ll never forget," she said. "It was like two trucks colliding.”

Niles was taken to New York-Presbyterian Hospital in stable condition, an NYPD spokesman said.

There was no immediate information on what sparked the stabbing, according to police.

The father is reportedly a founding member of the accounting firm Citrin Cooperman.

"We have no comment on this," said a woman who answered the phone at the firm before hanging up.

Mark had been working as a host at Gallagher's Steakhouse in Midtown since February.

"It was with great sadness that we learned of the tragic death of our friend and colleague Mark Citrin," a spokesman from the restaurant said in a statement. "He was a smart, collegial, and delightful person with a bright future in the restaurant business. We mourn his loss and offer the deepest sympathy and condolences to his family."

Another coworker described Mark as a "nice kid.

"He was a positive influence on the restaurant. I was shocked," said the male coworker, who asked to remain anonymous. "I never thought he’d do something like that."

With reporting by Anton Nilsson.