A knife-wielding maniac was shot dead by police on Roosevelt Island early Saturday morning after he swung the weapon at officers inside an apartment building elevator, according to police sources.

The shooting in the typically quiet neighborhood unfolded after police arrived at 540 Main Street around midnight for a call about a man described by relatives as emotionally disturbed, sources said.

After the first group of cops entered one of the lobby’s elevators, the 21-year-old suspect emerged from the other elevator wielding a knife, according to sources.

He stepped into the lobby and confronted another officer who arrived as backup, swinging his knife at the cop, the sources said.

That cop retreated and the crazed man got into the other elevator, still occupied by the first group of cops to respond, and tried to stab the officers at close range.

The officers opened fire, killing the man.

A Post reporter who was walking her dog on the sidewalk outside the building at the time heard at least five gunshots.

A number of uniformed officers were seen running outside of the residential building before several shots were heard being fired into the lobby, the Post reporter observed.

Within roughly two minutes, NYPD vehicles started flying across the bridge, according to the reporter.

A witness told The Post they saw police coming out of the building in body armor.

Three ambulances and dozens of police cruisers were outside of the building after the shooting.