Firefighters battled flames on the sixth floor of 740 Park Avenue - located between East 71st and East 72nd streets.
Heavy smoke filled the sixth floor stairwell. Firefighters stretched one line from the stairwell. It's a 19-story building with 33 apartments.
Eyewitness News reporter Josh Einiger recorded this video at the scene:
The fire was knocked down by the evening, but there's still a smoke condition in the building.
Two firefighters suffered minor injuries while battling the fire.
The cause of the fire is under investigation.
The building, known as one of the city's "Tower of Power," is filled with wealthy and important residents, including businessmen David Koch, Ron Perelman, Ron Lauder and designer Vera Wang. It is the former residence of John D. Rockefeller and the childhood home of Jacqueline Kennedy Onassis.