Lamont Mason, 20, was outside 2400 Second Ave., near East 124th Street, about 4:51 p.m. when three men shot him in the chest and legs, an NYPD spokesman said.
Neighbors heard about five shots, they said.
"As soon as I heard that, I knew it was over," said Angel Rodriguez, 61.
Rodriguez went out to the scene and saw a crowd forming around the victim, he said.
"I saw them pumping his chest. I knew he wasn't going to make it," Rodriguez said.
"A young guy, just a young kid."
Paramedics found Mason lying on the floor face up with several gunshot wounds. He was pronounced dead at Harlem Hospital, police said.
Surveillance footage shows him getting into a fight nearby earlier in the day, an NYPD spokesman said. Investigators believe the two altercations are linked.
The shooters fled and there were no immediate arrests, police said.
Monday morning friends and neighbors mourned Mason by creating a makeshift memorial in front of the building he lived. Empty bottles, lit candles, and the blood-stained defibrillator package from the device used to try to save his life decorated the memorial.
"He grew up here," said a friend who did not want to disclose her name. "He was a good kid, he didn't do anything to deserve this."
Friends wrote messages like , "Gone but never forgotten," and "Sleep easy Lil Bro," on the wall of the building's entrance.
"Whoever did this better not show their face," another friend said.