It happened 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.
The victim was sunbathing alone in a quiet, 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.
"That's a really peaceful part of the park and I've not heard of anybody having problems. I hang out there at night," said parkgoer Tricia Regan.
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 are also using drones to help comb through every inch of the park.
They describe the suspect as a man in his 30s, about six feet tall, with a medium build and pearly hair, wearing a light-colored shirt and shorts.
There were no cameras in that particular area, but there are many throughout the park, so police are going through cameras hoping the suspect was captured somewhere and the victim might recognize him.
Police say the victim will be working with a sketch artist to get an image of the suspect released out to the public.
"We're going to keep Central Park safe. And we're going to continue to be out here until we feel that this perpetrator is off the street," said NYPD Deputy Commissioner Tariq Shepherd.
No arrest has been made and the investigation is ongoing.
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