It happened inside Central Park near 104th Street and Central Park Drive around 1:30 p.m.
Police said when they arrived they were informed that a man sexually assaulted a 21-year-old woman.
NYPD gives update on Central Park sex assault
The victim was sunbathing alone in a grassy area, called the Great Hill section, when the suspect pulled down his pants and tackled her, police said.
She fought the man off, screamed and ran away.
The suspect fled the scene southbound down West Drive.
The victim was canvassing the park with police in a cruiser to help look for the suspect.
She is now at a local hospital with NYPD Special Victims detectives.
Top police officials are in the park leading the search.
"All our strategic response groups are converging in this park right now," NYPD Chief John Chell said. "Right now we're searching through each part of this park for witnesses or any evidence you might find. So we're cutting the park into pieces, everyone's got a piece and they're going out to get eyewitnesses."
Police described the suspect as a man in his 30s, about six feet tall, with a medium build and curly hair, wearing a light-colored shirt and shorts.
Police say the victim will be working with a sketch artist to get an image of the suspect released out to the public.
No arrest has been made and the investigation is ongoing.